Carbon Isotope Chemostratigraphy and Sea

Geological Magazine Carbon isotope chemostratigraphy and sea-

www.cambridge.org/geo level history of the Hirnantian Stage (uppermost Ordovician) in the Oslo–Asker district, Norway

1 † 2,3 Original Article Mikael Calner , Johan Fredrik Bockelie , Christian M.Ø. Rasmussen , Hanna Calner1, Oliver Lehnert4 and Michael M. Joachimski4 Cite this article: Calner M, Bockelie JF, Rasmussen CMØ, Calner H, Lehnert O, and 1Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden; 2Natural History Museum of Joachimski MM. Carbon isotope – 3 chemostratigraphy and sea-level history of the Denmark, University of Denmark, Øster Voldgade 5 7, DK-1350, Copenhagen, Denmark; GLOBE Institute, 4 Hirnantian Stage (uppermost Ordovician) in the University of Copenhagen, Copenhagen, Denmark and GeoZentrum Nordbayern, Friedrich-Alexander University Oslo–Asker district, Norway. Geological Erlangen-Nürnberg (FAU), Schlossgarten 5, D-91054, Erlangen, Germany Magazine https://doi.org/10.1017/ S0016756821000546 Abstract δ13 Received: 10 August 2020 We present a Ccarb chemostratigraphy for the Late Ordovician Hirnantian Stage based on Revised: 4 May 2021 208 whole-rock samples from six outcrops in the Oslo–Asker district, southern Norway. Our Accepted: 21 May 2021 data include the Norwegian type section for the Hirnantian Stage and Ordovician–Silurian Keywords: boundary at Hovedøya Island. The most complete record of the Hirnantian Isotope Carbon HICE; oolite; incised valley; glaciation; Excursion (HICE) is identified in a coastal exposure at Konglungø locality where the preserved brachiopods; Konglungø; Hovedøya part of the anomaly spans a c. 24 m thick, mixed carbonate–siliciclastic succession belonging to δ13 the upper Husbergøya, Langåra and Langøyene formations and where Ccarb peak values Author for correspondence: Mikael Calner, þ ‰ Email: [email protected] reach c. 6 . Almost the entire HICE occurs above beds containing the Hirnantia Fauna, suggesting a latest Hirnantian age for the peak of the excursion. The temporal development †J.F.B. passed away 7 October 2016. of the HICE in southern Norway is associated with substantial shallowing of depositional environments. Sedimentary facies and erosional unconformities suggest four inferably fourth-order glacio-eustatically controlled sea-level lowstands with successively increased exposure and erosion to the succession. The youngest erosional unconformity is related to the development of incised valleys and resulted in cut-out of at least the falling limb of the HICE throughout most of the Oslo–Asker district. The fill of the valleys contains the falling limb of the HICE, and the postglacial transgression therefore can be assigned to the latest part of the Hirnantian Age. We address the recent findings of the chitinozoan Belonechitina gamachiana in the study area and its relationship to the first occurrence of Hirnantia Fauna in the studied sections, challenging identification of the base of the Hirnantian Stage.

1. Introduction The Hirnantian Age in the latest Ordovician is of wide international interest due to its association with the peak of the early Palaeozoic Icehouse (EPI; Page et al. 2007), the Late Ordovician Mass Extinction (LOME; Finnegan et al. 2012; Harper et al. 2014; Ling et al. 2019; Rasmussen et al. 2019; Wang et al. 2019) and an associated major anomaly to the global carbon cycle, the Hirnantian Isotope Carbon Excursion (HICE; Brenchley et al. 2003; Kaljo et al. 2008; Bergström et al. 2013; Subías et al. 2015; Mauviel & Desrochers, 2016). The possibility of assessing the Hirnantian Stage and demonstrating the precise interrelationship of stage boundaries, biostratigraphy, sea-level trends and the temporal development of the HICE is hampered by the global rarity of complete sections: the HICE is to date recorded from several tens of localities world- wide, but few, if any, of the associated outcrops or core sections have a continuous sedimentary record due to the contemporaneous glacio-eustasy (e.g. Loi et al. 2010; Ghienne et al. 2014). Formally, the base of the Hirnantian is defined as the First Appearance Datum (FAD) of the graptolite Metabolograptus extraordinarius 0.39 m below the Kuanyinchiao Bed in the Wangjiawan North Section, Hubei province, China (Chen et al. 2006; Gorjan et al. 2012). © The Author(s), 2021. Published by Cambridge At the immediately adjacent Wangjiawan Riverside Section, the HICE starts very close to University Press. This is an Open Access article, the FAD of M. extraordinarius (Gorjan et al. 2012). Recent U–Pb dates from a continuous sec- distributed under the terms of the Creative tion at Wanhe in southwest China suggest that the duration of the entire Hirnantian Age is as Commons Attribution licence (http:// creativecommons.org/licenses/by/4.0/), which little as 0.47 ± 0.34 Ma, and thus considerably shorter than previously understood (Ling et al. permits unrestricted re-use, distribution and 2019). Bergström et al. (2009) subdivided the Hirnantian Stage into two stage slices, named Hi1 reproduction, provided the original article is and Hi2. Hi1 is defined as strata between the base of the Metabolograptus extraordinarius properly cited. Graptolite Zone and the end of HICE. Hi2 is thus defined as strata between the end of the HICE and the base of the Akidograptus ascensus Graptolite Zone, marking the top of the Ordovician. In Baltoscandia the HICE has been particularly studied from sections in the East Baltic area (Kaljo et al. 2001, 2004a, 2007, 2008, 2012; Ainsaar et al. 2004, 2010; Meidla et al. 2004; Hints

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et al. 2010, 2014; Bauert et al. 2014; Ainsaar, 2015). Important data unconformity below the Silurian deposits. Brenchley & have also emerged from Sweden (Schmitz & Bergström, 2007; Newall (1975) studied the sedimentology and palaeontology Bergström et al. 2012, 2013; Ebbestad et al. 2015) where the pre- of the Katian through Hirnantian and produced palaeogeo- served Hirnantian Stage is normally only a few metres thick and graphical maps for the upper Fjord area. They showed that associated with marked disconformities (e.g. Bergström et al. the facies pattern in Oslo–Asker was complex, with an ‘eastern’ 2012, 2013). and ‘western’ facies subdivision; the western parts being richer δ13 This study aims to establish a Ccarb chemostratigraphy in carbonates whereas siliciclastics are more important in the for the Hirnantian Stage of the Oslo–Asker district of eastern part. Johnson & Baarli (2018) further elaborated on this Norway. This region represents the margin of Baltoscandia subdivision and showed that, in the late Hirnantian, higher land and faced the closing Iapetus Ocean towards the west during areas existed in the northwest, whereas marginal marine areas theearlystagesoftheCaledonianOrogeny.Wepresentδ13C were found to the southeast, and that incised valleys were situ- data from six outcrops (Fig. 1;Appendix1), including from ated between these areas. The stratigraphic interfingering of the the Norwegian type section for the Hirnantian Stage and the ‘eastern’ and ‘western’ facies is obvious in central positions Ordovician–Silurian boundary at Hovedøya Island and from where distinct lithostratigraphic units therefore are recurrently an incised valley fill that represents the ‘missing strata’ at a occurring in a stratigraphic section; this is exemplified in the major unconformity resulting from Hirnantian glacio-eustasy present study. (Bockelie et al. 2017). Our primary goal has been to identify the A recent paper by Bockelie et al.(2017) presents a historic back- HICE in order to understand the degree of preservation of ground to the successive improvements in understanding that have Hirnantian strata, and thus illustrate the stratigraphic com- taken place with regard to the Ordovician–Silurian boundary inter- pleteness of the exposed successions in the area. This is par- val in the area, and also provides a detailed description and partial ticularly warranted as Norway is largely a white spot on the revision of the stratigraphy of these rocks. Their revised stratigra- map in terms of Ordovician carbon isotope stratigraphy. phy is followed in the present paper (Fig. 2). Bergström et al. (2010) documented the Katian Guttenberg Carbon Isotope Excursion (GICE) from the Nes-Hamar district north of Oslo. Apart from this, only a few previous studies 3. Material and methods have shown a partial record of the upper Katian and/or Hirnantian stages from localities in the Oslo–Asker district For this paper we have studied and sampled five formations span- (Brenchley et al. 1997; Brenchley & Marshall, 1999; Kaljo ning the upper Katian through lowermost Silurian of the Oslo– et al. 2004b; Bergström et al. 2006;Amberget al. 2017). Asker district: the Skogerholmen Formation, the Husbergøya Shale Formation, the Langåra Limestone–Shale Formation, the Langøyene Sandstone Formation and the Solvik Formation 2. Geological setting and stratigraphy (Fig. 2). M.C. and H.C. studied natural outcrops at six locations At the end of the Ordovician period, present-day southern Norway during field seasons 2014–16, and J.F.B. participated in two of these was situated near latitude 30°S (Scotese & McKerrow, 1991; fieldworks. The studied localities include the southwestern shore of Torsvik & Cocks, 2013). The Ordovician rocks exposed in and the island Hovedøya (59° 53 0 30.01″ N, 10° 43 0 52.43″ E), Pilbogen around Oslo city and in the many islands in the upper Oslo Bay in western Brønnøya (base of section at 59° 51 0 10.90″ N, 10° Fjord area form part of a fault-controlled graben with a c. 31 0 34.28″ E; top of section at 59°51 0 11.32″ N, 10° 31 0 33.95″ E), 2500 m thick lower Palaeozoic succession (Bruton & Williams, Store Ostsundet, more specifically near the intersection of 1982; Nakrem & Rasmussen, 2013). The Ordovician part of this Vierndroget and Viernveien in eastern Brønnøya (59° 51 0 succession is c. 400 m thick and comprises fossiliferous limestone 47.24″ N, 10° 33 0 20.40″ E), Konglungø (base of section at 59° alternating with units of fine-grained siliciclastic strata (Bockelie, 50 0 24.46″ N, 10° 30 0 53.72″ E; top of section at 59° 50 0 22.77″ 1978, 1982; Owen et al. 1990). The study of these rocks, and N, 10° 30 0 54.17″ E), Vettre road-cut (59° 49 0 47.35″ N, 10° 28 0 reconstruction of depositional environments and palaeogeography 11.54″ E) and Olledalen skytebane (Olledalen shooting range, of the area, is complicated by crustal shortening related to the 59° 50 0 45.22″ N, 10° 22 0 47.61″ E; Fig. 1). δ13 Caledonian Orogeny and associated nappe displacement, resulting A total of 208 Ccarb whole-rock samples were collected from in folding and thrusting of the pre-orogenic strata (Bruton et al. these localities using a handheld micro-drill. The majority of our car- 2010). This shortening is very clear in the Oslo area, where a large bon isotope data derive from Konglungø (86 samples) and Hovedøya number of anticlines and synclines with steep limbs strike from the (49 samples), and for this reason these localities are discussed in southwest towards the northeast. greater depth. Note that Konglungø is a protected locality The stratigraphy of the Hirnantian Stage and the (‘Naturminne’) and private property that requires a special permit Ordovician–Silurian transition in the Oslo area has been studied from both landowner and FylkesmanneninVikencountytosample. δ13 for more than 150 years, and several key papers have succes- All Ccarb analyses were carried out at the GeoZentrum of sively contributed to a better understanding of the interval Northern Bavaria of the Friedrich-Alexander University (Kiær, 1902, 1908;Spjeldnæs,1957;Brenchley&Newall, Erlangen–Nuremberg (Germany). The carbonate powders were 1975, 1980;Brenchley&Cocks,1982; Brenchley & Marshall, reacted with 100 % phosphoric acid at 70 °C using a Gasbench 1999;Bockelieet al. 2017; Johnson & Baarli, 2018). Folding of II connected to a ThermoFisher V Plus mass spectrometer. All val- the succession in combination with scattered distribution of ues are reported in per mil relative to V-PDB. Reproducibility and the rocks on the many islands, however, still makes a full under- accuracy of carbon isotope analyses was monitored by replicate standing of especially the boundary interval challenging. The analysis of laboratory standards calibrated by assigning δ13C values immediate Ordovician–Silurian boundary succession was stud- of þ1.95 ‰ to NBS19 and −46.6 ‰ to LSVEC. Reproducibility ied in detail along the southwestern shore of Hovedøya by of the δ13C analyses was in the range ±0.02 ‰ to ±0.04 ‰ (± 1 Spjeldnæs (1957) who concluded that there was an angular std dev.).

Fig. 1. Map showing the location of the six sampled outcrops (names in italics) in the Oslo metropolitan area, southern Norway. The two localities on Hovedøya refer to the Ordovician–Silurian type section (the SE locality; Brenchley & Newall 1975) and the locality where the incised valley was studied (the SW locality).

4. Results lower shale unit with few, laminated siltstone beds, interpreted as storm deposits (tempestites), which often have a slightly calcareous 4.a. Stratigraphy and sedimentary facies at Konglungø upper part (Fig. 4c). The siltstone beds become more frequent and The Konglungø locality constitutes well-exposed coastal sections thick up-section, suggesting a slight progradation of the depositio- displaying Katian through lower Silurian strata, with gravelly nal system. The lower part of the upper half of the formation is beach deposits covering only minor parts of the succession covered by modern beach deposits and could not be assessed. (Fig. 3). The strata represent the southern limb of an anticlinal that Above this level in the upper parts of the formation, nodular lime- strikes SW–NE, and strata are overturned to a near-vertical posi- stone beds become successively more important and here the facies tion. The exposed strata become successively younger to the south- resembles that of the lower Skogerholmen Formation (unit 4dα). east and belong, in ascending order, to the Skogerholmen, This up-section increase in calcareous facies is restricted to the Husbergøya, Langåra, Langøyene and Solvik formations (Figs Asker district and does not occur at the type section for this for- 3–9). The Skogerholmen Formation (Owen et al. 1990, p. 33) out- mation at Husbergøya (Brenchley & Newall, 1975, p. 256). crops in the northernmost end of the section and can be subdivided Towards the top of this unit at Konglungø a rich shelly fauna into three subunits based on lithology, in ascending order referred appears (the first specimens at −21 m in the measured profile; to as 4dα,4dβ and 4dγ (sensu Størmer, 1953; Fig. 4a–b). These Fig. 8). The topmost c. 2 m of the Husbergøya Formation is devel- units, originally part of a local stage (etasje) nomenclature, were oped as a distinct unit of brown siltstone to fine-grained sandstone later renamed as the Hovedøya Member (4dα) and the (Fig. 4e; unit D2 in Figs 3, 9) that is thoroughly bioturbated and Spannslokket Member (4dβ and 4dγ) by Owen et al. (1990) but homogenized. This unit was recognized at several localities by are included here also in their original form for the sake of detail. Brenchley & Newall (1975, figs 6–7) and is a good marker bed Only the uppermost c. 7 m of 4dα is exposed at Konglungø in the upper Oslo Fjord area. It is herein referred to as the brown (Fig. 4a). Subunits 4dα (7þ m) and 4dγ (9.4 m) constitute nodular bioturbated sandstone unit to separate it from other brown silt- and to continuous bands of limestone alternating with shale, whereas sandstone units in the succession that are mentioned in this text. the intervening 4dβ (5.4 m) is composed of shale with a few lami- Brenchley & Newall (1975, pp. 258–9) reported Tretaspis sp. from nated siltstone beds (in old literature the ‘Isotelus Shale’). The this unit. Later, Brenchley & Cocks (1982, p. 786 and fig. 4) noted boundary between 4dβ and 4dγ is taken at the level for the upper- that Tretaspis had its last appearance datum (LAD) within the most (youngest) siltstone bed in this part of the section. The brown bioturbated sandstone unit and reported an abundant Skogerholmen Formation is sharply overlain by the Husbergøya Hirnantia Fauna from the very top of the same unit at one locality Formation. The latter formation was defined by Brenchley & in the Oslo Fjord. Based on these findings they placed the base of Newall (1975) and is a shale-dominated succession with an the Hirnantian Stage there. The brown bioturbated sandstone unit, increasing number of calcareous, silty to sandy beds up-section. which is fairly calcareous at Konglungø (but still easily identified), At Konglungø the formation is 24 m thick. It constitutes a distinct is sharply overlain by a 1.55 m thick, conspicuously light-grey

Fig. 2. Upper Ordovician and lower Silurian stratigraphic nomenclature of the Oslo–Asker district as revised by Bockelie et al.(2017). The grey numbers in the formations column mark the thickness of the formations in their respective type section (from Owen et al. 1990). The older local stratigraphic subdivision of the Skogerholmen Formation (4dα,4dβ and 4dγ) is based on Størmer (1953), whereas the local stages 5a–b were introduced by Kiær (1902) and 5c by Spjeldnæs (1957). BBSU = brown bioturbated sandstone unit (an informal but useful marker bed in the uppermost Husbergøya Formation). See Bockelie et al.(2017) for an extended discussion of the Hirnantian stratigraphy of the area.

limestone unit with abundant shelly fauna, including solitary et al.(2017). Between 1.25 and 1.65 m below the top of the lower streptelasmatid corals, brachiopods, crinoids, bryozoans and the sandstone unit is a sandstone bed with abundant slump structures cystoid Heliocrinites sp., sometimes in dense allochthonous con- (Fig. 6b). The upper sandstone unit (C1) is 5.3 m thick and con- centrations (Figs 4f and 5; unit D1 in Figs 3, 9; see also stitutes thick brown-coloured beds of amalgamated hummocky Neuman, 1969). It also includes abundant, sub-centimetre-sized, cross-stratified sandstone beds, i.e. it lacks the abundant shale beds reworked clasts of a dark fine-grained lithology implying nearby that typify the lower sandstone unit. This unit is interpreted as cor- sea-floor erosion. This limestone belongs to the Langåra responding to the Høyerholmen Member sensu Bockelie et al. Formation, which has a westerly distribution in the Oslo–Asker (2017). A second distinct sandstone bed with slump structures district (Brenchley & Newall, 1975; Bockelie et al. 2017), but which occurs at the base of this upper sandstone unit. The calcareous unit interfingers with the sandy Langøyene Formation at Konglungø. (C2) separating the two sandstone intervals is, again, the expres- Based on its lithology and abundant fossil content, this light-col- sion of interfingering with the more westerly Langåra Formation oured limestone unit (D1) stands out as a higher-energy shallow- (Fig. 6c). The upper sandstone unit is sharply overlain by a water deposit in the overall fine-grained and clastic-dominated light-grey-coloured, c. 3 m thick and trough cross-bedded oolitic facies of the Konglungø section. Its lower half is composed of mas- and strongly calcareous sandstone unit (Fig. 7a). This is the sive beds of packstone and grainstone whereas the upper half is Pilodden Member of the Langøyene Formation (sensu Bockelie interbedded with thin shale beds (Fig. 4e). Macrofossils of the unit et al. 2017) and unit B in Figures 3 and 9. It varies from being are disarticulated and relatively worn. The top of the limestone an oolitic sandstone to a sandy oolitic limestone or bioclastic grain- marks a pronounced shift to coarser-grained clastic deposition. stone and forms a distinct marker bed throughout the Oslo–Asker These siliciclastics belong lithostratigraphically to the Langøyene district. It attains a maximum thickness of c. 9 m in the Formation and form two sandstone intervals separated by a northeastern part of Brønnøya (at 59° 52 0 1.46″ N, 10° 32 0 c. 2 m thick unit of argillaceous limestone and marl (units 52.28″ E). The Pilodden Member is commonly trough cross- C3–C1 in Figs 3, 9). The lower sandstone unit (C3) is 6 m thick bedded with sharp, erosive lower and upper contacts. This is the and characterized by thin to medium-thick hummocky cross- case also at Konglungø where the scoured base of the oolitic sand- stratified sandstone beds (storm surge deposits of Brenchley stone includes scattered reworked clasts of the underlying sand- et al. 1979; Fig. 6a) interbedded with thin shale beds. It is inter- stones (Fig. 7b). The top of the oolitic sandstone at Konglungø preted as corresponding to the Skaueren Member sensu Bockelie is overlain by a few decimetres thin, coarse-grained unit of pure

Fig. 3. Sketch showing the stratigraphy at the most expanded section sampled for this study: the eastern end of Konglungø. (Note that access to this private property and sampling here require permission.) The coding from A to F refers to stratigraphic subunits used during field work and therefore may or may not overlap with formal stratigraphic units. They are discussed in the text and are also shown in the photoplates and sedimentary profiles from Konglungø and Brønnøya to facilitate detail (Figs 8–9 and 11). Based on brachiopod faunas and the carbon isotope chemostratigraphy presented herein, the Katian–Hirnantian boundary is drawn in the higher parts of the Husbergøya Formation. BBSU = brown bioturbated sandstone unit.

quartz arenite. The quartz grains are well-rounded, sub-spherical position of the Ordovician–Silurian boundary is uncertain at this and sometimes several millimetres in size and were referred to as locality, but the presence here of the conodont Ozarkodina oldha- ‘millet-seed’ sandstone facies by Brenchley & Newall (1975, p. 263; mensis 8 m above the base of the Solvik Formation implies that the see also Fig. 7d). This coarse-grained veneer scours down in the base of the Silurian System is within the lower 8 m of the formation underlying oolitic sandstone and includes frequent heavily at this locality (Aldridge et al. 1993; Baarli, 2014, p. 30). reworked clasts from that bed. The top surface of the oolitic sandstone is therefore interpreted as an erosional unconformity and the 4.a.1. Carbon isotope stratigraphy at Konglungø and overlying ‘millet-seed’ sandstone unit as a basal transgressive lag chitinozoan biostratigraphy deposit (this interpretation is further supported by an offset in δ13C Eighty-six carbon isotope samples were taken at Konglungø values of more than 2 ‰ across this boundary; see below and (Figs 8–9; Appendix 1). The carbon isotope stratigraphy presented Fig. 9). The ‘millet-seed’ sandstone unit is in turn overlain by a is not complete, due to the partial coverage of the succession by yellowish-buff-coloured limestone bed (named ‘kaki-bed’ in modern beach deposits, but still provides the best record so far Fig. 7). This bed is intriguing because it is a deviating facies to for correlation of the strata and for a chemostratigraphic identifi- the entire Konglungø section and signals a total stop of influx of cation of the Hirnantian in Norway. Even if there are abundant coarser siliciclastics to this part of the basin. It represents a brief shaly intervals in the succession, most of it has interlayered, slightly period of carbonate production and preservation before further argillaceous micritic limestone or even clean limestone from which rapid deepening of the depositional environment reflected by the samples were taken. This was the case for the Skogerholmen the black shale of the overlying Solvik Formation (note that and much of the Husbergøya formations. In the lowermost Baarli (2014, fig. 3) formally included this limestone bed in the Solvik Formation, samples were taken from thin limestone beds Solvik Formation). The lower Solvik Formation includes a few, thin (brachiopod coquinas) interlayered in the shale. The two calcare- limestone beds, notably also a few brachiopod coquinas just above ous sandstone units in the Langøyene Formation include primary the base of the formation at Konglungø (Fig. 7c). The exact skeletal debris and are fairly calcareous in the Asker district;

Fig. 4. (Colour online) Photoplate showing stratigraphy and lithology of the lower parts of the Konglungø section. (a) The lowermost portions of the section at Konglungø exposing the Spannslokket Member of the Skogerholmen Formation, with further stratigraphic subdivision in 4dα,4dβ and 4dγ (sensu Størmer, 1953). The transition from 4dα to 4dβ marks a deepening of the depositional environment, a decrease of carbonate deposition and a 1 ‰ lowering of δ13C values that may represent a part of either the ‘lower HICE’ or the Paroveja excursion (see text). (b) Limestone–marl alternation of the uppermost Skogerholmen Formation (4dγ) abruptly overlain by shale of the lower Husbergøya Formation. This transition from limestone–marl to shale can be traced throughout the Oslo–Asker district (Brenchley & Newall, 1975) and marks a distinct transgression. (c) Lower parts of Husbergøya Formation showing shale succeeded by a gradual up-section increase in carbonate content. The lighter, carbonate-rich bands represent mixed carbonate–siliciclastic distal tempestites. (d) Detail of the middle Husbergøya Formation showing rhythmic bedding of calcareous shale and tempestites. (e) Stratigraphy of the early Hirnantian. The brown bioturbated sandstone unit in the top part of the Husbergøya Formation is c. 2 m thick at Konglungø and is a good marker bed in the area (see Brenchley & Newall, 1975). It is rich in shallow-water fossils, notably corals. The rising limb of HICE is identified in the upper Husbergøya Formation and reaches δ13C values between 3 and 4 ‰ in this brown bioturbated sandstone unit (5.15 ‰ in the very top; sample KON66). Also the LAD of Tretaspis and the FAD of Hirnantia Fauna is within this unit (see text). The overlying limestone unit represents the Langåra Formation and marks the end of the rising limb of HICE with all values above 5 ‰ and a top value in its uppermost part of 6.01 ‰ (KON35). Above the limestone is a sharp transition to calcareous sandstone facies of the Langøyene Formation. (f) Solitary corals in the brown bioturbated sandstone unit.

Fig. 5. (Colour online) Polished slab showing abundant solitary streptelasmatid corals, large brachiopods and crinoidal debris in a bioclastic grainstone. This bed derives from the lower portion of unit D1, representing the only clean limestone interval in the succession at Konglungø (interfingering of the westerly Langåra Formation) and which marks shallowing above the fair-weather wave base. Its deposition corresponds to the end of the rising limb of HICE at Konglungø with δ13C values between 5 and 6 ‰. The width of the bed is c. 15 cm.

Fig. 6. (Colour online) Photoplate showing stratigraphy and lithology of the middle parts of the Konglungø section, including divisions of tempestites (sensu Brenchley, 1989). (a) 30 cm-thick hummocky cross-stratificated, calcareous sandstone of the lower tempestite unit, Skaueren Member in the Langøyene Formation. The thickness of the tempestites and the preservation of only the lower divisions of idealized tempestite sequences suggest rapid deposition in relatively shallow waters, just below the wave base. (b) Calcareous sandstone with slump structures suggesting reworking of sands on relatively steep gradients, such as in a delta front environment. (c) Limestone–marl alternation between the lower and upper sandstone units, representing interfingering of the westerly Langåra Formation.

Fig. 7. (Colour online) Photoplate showing stratigraphy and lithology of the upper parts of the Konglungø section. (a) Transition from the upper Langøyene Formation to the Silurian Solvik Formation. The amalgamated storm beds of the upper tempestite unit (Høyerholmen Member) are overlain sharply by oolitic sandstone (Pilodden Member). The latter unit is a widespread marker bed in the upper Oslo Fjord, its maximum thickness reaching c. 9 m at Brønnøya. (b) Detail of the unconformable transition from tempestite facies to oolitic sandstone showing larger-scale trough cross-bedding. (c) The top of the Langøyene Formation at Konglungø. The upper boundary of the oolitic sandstone is overlain by a thin unit of unusually coarse quartz grains – the ‘millet-seed’ sandstone of previous authors. This small unit also contains several clasts reworked from the underlying oolitic sandstone. These clasts along with a drop of δ13C values exceeding 2 ‰ between the oolitic sandstone and the ‘millet-seed’ sandstone unit imply erosion below the latter unit. The ‘millet-seed’ sandstone unit thus is a transgressive lag deposit. It is overlain by a thin calcareous bed referred to herein to as the ‘kaki bed’ due to its kaki colour. (d) Detail of the ‘millet-seed’ sandstone unit above the eroded oolitic sandstone.

however, that part of the curve may partly reflect precipitation of Fauna. It would also suggest – as inferred based on the data pre- cement during the early diagenesis. sented here – that the main part of the Skogerholmen and The lower portions of the section, corresponding to the middle Husbergøya formations belonged in the M. extraordinarius and upper Skogerholmen Formation through the middle parts of Graptolite Zone, since high δ13C values globally appear to be the Husbergøya Formation, have δ13C values between 0 and 1 ‰. restricted to the M. persculptus Zone. There is one deviation from these baseline values that may be of Another option is that the higher δ13C values at the top of the importance for at least regional correlation of the strata, namely Hovedøya Member reflect the falling limb of the Paroveja excur- the distinctly higher δ13C values in the top of the Hovedøya sion of Ainsaar et al. (2010). This small positive excursion is Member (unit 4dα) of the Skogerholmen Formation (e.g. samples known from the East Baltic area where δ13C values reach KON76: 1.39 ‰; and KON74: 1.55 ‰). The corresponding strata between 1.5 and 2 ‰ (very similar to the ‘lower HICE’)and at Hovedøya include the chitinozoan Belonechitina gamachiana form isotopic zone no. 14 (BC14; see Ainsaar et al. 2010). It (Amberg et al. 2017), which in North American sections has been is, however, associated with the chitinozoan Conochitina rugata reassigned to the lowermost Hirnantian (Delabroye & Vecoli, and the eponymous biozone and thus distinctly older than the 2010; Amberg et al. 2017). The higher δ13C values in the top of B. gamachiana Biozone (cf. Nõlvak et al. 2006;Amberget al. the Hovedøya Member would then potentially reflect the falling 2017). Future sampling of the lowermost c. 3moftheexposed limb of the ‘lower HICE’ identified in Anticosti Island, Canada strata at Konglungø, and at the type section for the (Mauviel & Desrochers, 2016), and possibly in Mirny Creek, Skogerholmen Formation, may help to further constrain the Siberia (Kaljo et al. 2012). The implications in the studied area relationship between B. gamachiana and carbon isotope stratig- would be significant as the preserved Hirnantian succession would raphy and thus clarify this stratigraphic problem. then be more than 60 m thick. Further, the base of the Hirnantian In the basal Spannslokket Member (unit 4dγ), sample KON83 would be situated more than 40 m below the FAD of the Hirnantia represents the stratigraphic level of the lowest δ13C value of the

Fig. 8. Sedimentary log profile and carbon isotope stratigraphy of the lower and middle parts of the Konglungø section. The lowermost five samples (KON72-KON76) may represent the falling limb of the ‘early HICE’ carbon isotope excursion or, less likely, the Paroveja excursion. The start of HICE is herein defined as at sample KON47, and the remainder of the section constitutes the rising limb of the HICE (see text for discussion). Note that the chitinozoans B. gamachiana and S. cf. taugourdeaui are approximated into the section based on findings in Hovedøya (Amberg et al. 2017). These levels are confidently correlated, however, based on the very similar facies development and carbon isotope data.

whole Konglungø section (0.11 ‰) and reflects the turning point to of the HICE and we assign the start of it to sample KON47 in constantly increasing δ13C values that reach slightly more than 1 ‰ Figure 8. The most rapid rate of change in δ13C values takes place in the top of the Skogerholmen Formation. δ13C data are lacking between c. −23 m and −21 m in the section (thus very close to the from the shales of the lower Husbergøya Formation. The next few ‘rusty spike’ in the section, marked in Figs 3 and 8). Here δ13C val- metres of the section are covered by beach deposits, and no isotope ues increase by c. 1 ‰ per metre section. data exist. Higher up in the more limy parts of the formation, δ13C In the fossiliferous packstone and grainstone of the Langåra steadily increases to values of almost 5 ‰ in the uppermost part of Formation (unit D1), δ13C reaches peak values between c. 5.5 the Husbergøya Formation. We interpret this rise as the rising limb and 6 ‰ (Fig. 9). The top of the limestone marks a major change

2 ‰ over a bedding plane (Fig. 9). Based on both sedimentology and carbon isotope stratigraphy, the unconformity is at the top of the oolitic sandstone unit and overlain by the thin but coarse- grained, dark sandstone unit referred to as the ‘millet-seed’ sandstone unit in Figures 7c–d and 9. The millet-seed sandstone unit is overlain by the ‘kaki bed’ in which two samples gave δ13C values of c. 1.5 ‰ and 1.9 ‰, respectively. The kaki bed is overlain by black shale of the Solvik Formation. A few limestone beds (brachiopod coquinas) occur in the lowermost metre of the Solvik Formation. The two lowermost of these have δ13C values of 0.96 ‰ and 0.83 ‰ (KON9 and KON10, respectively), suggesting a Silurian age. The next and highest sampled bed for this study, however, shows a distinct increase to 1.7 ‰ (KON11).

4.a.2. Biostratigraphical inference at Konglungø based on brachiopods As the geological setting of the Oslo–Asker district is tectoni- cally complex, stratigraphical correlations are sometimes difficult to make even within short distances. The studied succession, unfortunately, lacks graptolites, making it more difficult to correlate to global events. Some levels, though, are rich in shelly fossils, and brachiopod faunas in particular have tradi- tionally been applied for resolving stratigraphical issues. The following stratigraphic data are largely owed to the information provided by Brenchley & Cocks (1982)andCocks(1982). Brenchley & Cocks (1982) conducted a thorough palaeoecolog- ical study across the Hirnantian successions in the Oslo–Asker district, including the Konglungø section (their ‘Konglungen E 16’ locality; e.g. their fig. 3), in which they assigned laterally distin- guishable brachiopod associations. Bockelie et al.(2017) resolved the regional Hirnantian lithostratigraphy, enabling an interpretation that placed the brachiopod associations of Brenchley & Cocks (1982) in biostratigraphically discernible patterns across the region. As some of the faunas were found specifically at Konglungø – the main locality of the current study – we here place our main section into this biostratigraphical context. Towards the top of the Husbergøya Formation, the Onniella Association is developed, which along with the trilobite Tretaspis contains the brachiopod species Onniella kalvoya, Eoplectodonta oscitanda, as well as a species of Nicolella. Partly Fig. 9. Sedimentary log profile and carbon isotope stratigraphy of the upper parts of overlapping and succeeding this fauna is a moderately diverse the Konglungø section. The vertical repetition of Langåra and Langøyene formations is fauna assignable to the Hirnantia Fauna, with typical elements due to interfingering of westerly carbonate (Langåra) and easterly siliciclastic such as H. saggitifera, Dalmanella testudinaria, along with (Langøyene) facies in this area. The δ13C values remain high throughout the Cliftonia aff. psittacina, Eostropheodonta hirnantensis and δ13 Langøyene Formation. This is the plateau of the HICE. Note the offset in C values Hindella cassidea (Brenchley & Cocks 1982, p. 795). This fauna, of more than 2 ‰ at the top of the Pilodden Member, marking a major unconformity. Strata holding the falling limb of the HICE are clearly eroded along this unconformity. which historically denotes the lower-middle Hirnantian (Harper et al. 2014), occurs at the top of the Husbergøya Formation and continues a few metres into the overlying Langøyene Formation. in sedimentation pattern and a rapid transition to coarser siliciclas- In a recent review, however, Rong et al. (2020) demonstrated that tic tempestites of the Langøyene Formation. The δ13C record the Hirnantia Fauna is indeed diachronous, reaching into the M. shows less good stability through the overlying subunits (C3, C2 persculptus Graptolite Zone in places. Our isotopic data support a and C1); something that likely relates to the rhythmic sedimenta- late Hirnantian age for these strata. According to Brenchley & tion and high clastic influx during the corresponding time interval. Cocks (1982), this fauna is partly overlapping with the It is possible, however, to see a weak falling trend in the δ13C values Cliftonia–Hindella Association, which is best developed in the within this interval. more limestone-dominated western part of the outcrop area, cor- The erosional contact between the amalgamated sandstone responding to the two wedges of the Langåra Formation at beds of the Høyerholmen Member (unit C1) and the overlying Konglungø. The latter fauna consists of Hindella cassidea, oolitic quartz arenite of the Pilodden Member (unit B) is not asso- Cliftonia aff. psittacina, Eostropheodonta hirnantensis and ciated with any marked offset in the δ13C record (Fig. 9). The top of Triplesia sp., among others. The Dalmanella Association is a the oolitic unit, however, clearly represents a stratigraphic gap of a reduced variety of the Hirnantia and Hindella–Cliftonia associa- more significant nature. A major part of the falling limb of the tions and is strongly dominated by Dalmanella testudinaria, HICE is cut out at this level, and isotope values drop by more than whereas all other brachiopod genera ‘occur sporadically and in

low numbers’ (Brenchley & Cocks, 1982, p. 799). The Dalmanella represent the same type of incised valley fill that is demonstrated Association occurs in the regressive facies immediately above the from Hovedøya (see below). Hirnantia Association in the Langøyene Formation. According to Rong et al. (2020), both the Cliftonia–Hindella and the Dalmanella 4.c. Stratigraphy and sedimentary facies at Hovedøya associations developed in the Oslo–Asker district may represent a low-diversity Hirnantia Fauna. The overturned strata along the southwestern shores of Hovedøya Lastly, the topmost strata of the Langøyene Formation contain (Fig. 1) were assigned the Norwegian type section for the base of – the oolitic Pilodden Member. Brenchley & Cocks (1982, pp. 802–4) the Hirnantian Stage and the Ordovician Silurian boundary by reported on abundant coquinas of Brevilamnulella kjerulfi and Brenchley & Newall (1975, p. 253; see also Kiær 1908; Thebesia scopulosa together with elements typical of the Spjeldnæs 1957; Bockelie et al. 2017). Here, the oolitic sandstone Edgewood Fauna (sensu Rong & Harper, 1988). These species were of the Pilodden Member, Langøyene Formation, is separated from found in either oolitic–bioclastic limestone, siltstone or sandstone, the overlying black shales of the Solvik Formation by a 0.6 m thick and in both the eastern and western part of the herein studied area. brown siltstone unit (Kalvøya Member of Bockelie et al. 2017) fol- The finds likely represent the Pilodden Member as well as strati- lowed by a 0.6 m thick nodular limestone unit (Brønnøya Bed of graphic levels within incised valleys cut in that member, thus the Bockelie et al. 2017;Fig.13a). These two units have been discussed Kalvøya Member (cf. Brenchley & Cocks, 1982). Both the previously with regard to both environmental significance and the – Brevilamnulella and the Thebesia associations are regarded as latest placement of the Ordovician Silurian boundary and have been ‘ ’ Hirnantian in age. loosely referred to as the transitional beds between the Langøyene and Solvik formations. Kiær (1908) placed the O–S boundary on the top of the brown siltstone unit, whereas 4.b. Stratigraphy and sedimentary facies at Brønnøya Worsley (1982) later placed it at the top of the nodular limestone. The Husbergøya, Langåra and Langøyene formations are fairly Based on the occurrence of Ordovician brachiopods at least 50 m well exposed along the shore at Pilbogen Bay in the western part into the Solvik Formation, Baarli (2014, p. 30, and pers. comm., – of the island of Brønnøya (Fig. 10). The measured succession here 2020) argues that the Ordovician Silurian boundary should be – starts in the upper Husbergøya Formation at a level that was placed far higher up in the east part of the Oslo Asker district, covered by beach gravel at Konglungø (corresponding to some- although there is to date no detailed faunal information published where between −31 and −24 m at that locality). The exposed strata from the sections to define a more precise level. ‘ are of the same thickness as at Konglungø, although the relative The stratigraphic and depositional meaning of the transitional ’ thickness of the constituent subunits differs. A notable similarity beds , i.e. the brown siltstone unit and the nodular limestone, is is the sudden appearance of a rich shelly fauna in the uppermost now understood due to the exceptionally detailed mapping of Husbergøya Formation, i.e. at the same level as at Konglungø and the southwestern shores of the island by J.F.B. (first published ‘ during the rising limb of the HICE. by Bockelie 2013, p. 82; Bockelie et al. 2017): when these transitional beds’ are traced a few hundred metres laterally from the section depicted in Figure 13a, towards the west along the present-day 4.b.1. Carbon isotope stratigraphy at Brønnøya shore, it is clear that they are no longer underlain by the oolitic Twenty-five carbon isotope samples were taken at Pilbogen Bay sandstone. Instead they are underlain by thinly bedded siltstone (Fig. 11; Appendix 1). The sampling was not as dense as at and limestone, and the oolitic sandstone only occurs as reworked Konglungø, but the rising limb of the HICE is nevertheless well clasts in a conspicuous conglomerate a few metres stratigraphically δ13 indicated. The lowermost C samples in the upper below the brown siltstone unit (Fig. 13b). The distance between – ‰ Husbergøya Formation scatter around 1.5 1.75 and thereafter this oolitic sandstone conglomerate and the brown siltstone unit ‰ rise steadily through the remainder of the formation to c. 4 then increases successively towards the west, and the size of con- (sample BR19) in the brown bioturbated sandstone unit at the glomerate clasts increases substantially, the biggest clast reaching δ13 ‰ top of the formation. C values increase further to 5.5 in more than 1.5 m in width (Bockelie et al. 2017; Fig. 14). Bockelie the higher parts of the interfingering Langåra Formation (subunit et al. (2017) showed that the strata between the conglomerate and C2), similar to at Konglungø. The middle parts of the section at the brown siltstone unit have the geometry of an incised valley fill, Pilbogen were not sampled, but the higher parts of the of which only the thin eastern flank or wing is represented at the Langøyene Formation including the overlying oolitic sandstone often-visited, classical type section. To the west the valley fill, which δ13 ‰ ‰ (Pilodden Member) yield C values above 4 and 5 , respec- may be partitioned into eight lithological units, cut at least 16 m tively, also in good correspondence with nearby Konglungø. into the deeper levels of the Langøyene Formation (Bockelie et al. 2017, fig. 9). Of biostratigraphical importance to the current 4.b.2. Store Ostsundet, southeastern Brønnøya study, unit 6 – which only outcrops above present-day sea-level in – A conglomerate that, based on lithologic similarity, derives from the eastern part of this palaeovalley contains abundant Hindella the oolitic sandstone in the topmost Langøyene Formation is cassidea, along with rhynchonellid brachiopods (Stegerhynchus?). exposed at Store Ostsundet on the southeastern shore of Towards the top of the overlying fossiliferous and 3.5 m thick unit Brønnøya (Fig. 1). A carbon isotope sample from an oolitic sand- 7, Hindella cassidea reoccurs, this time together with Thebesia sco- stone clast here (Fig. 10c) gives a δ13C value of 5.9 ‰ (sample pulosa, among other taxa. BD05, Fig. 12), which is consistent with the oolitic sandstone at Pilbogen Bay, Konglungø and Hovedøya. The stratigraphic origin 4.c.1. Carbon isotope stratigraphy at Hovedøya of this conglomerate is thus confirmed. The conglomerate and the Forty-nine carbon isotope samples were taken at Hovedøya: 23 falling δ13C trend in the immediately overlying strata at Store fromthetypesectiondepictedinFigure13a, and 26 from the Ostsundet imply that the conglomerate here represents reworked incised valley fill (Figs 15–16; Appendix 1). Our first δ13Crecord parts of the Pilodden Member and that the overlying strata from the type section clearly demonstrates the unconformity

Fig. 10. (Colour online) Photoplate showing stratigraphy and lithology of the lower parts of the section at Pilbogen bay and at Store Ostsundet, both at Brønnøya. (a) Measured section at Pilbogen bay in southwestern Brønnøya. The stratigraphy much resembles that at Konglungø, 1600 m to the southwest, with the rising limb of the HICE identified in the upper Husbergøya Formation in the left part of the photograph. As on Konglungø, the start of the HICE is associated with the appearance of abundant shelly fossils, including corals. (b) Trough cross-laminated sandstone overlain by thick-bedded hummocky cross-stratified sandstone (upper tempestite unit of the Høyerholmen Member, Langøyene Formation). (c) Conglomerate clasts (arrows) of oolitic sandstone embedded in fine clastic rocks at Store Ostsundet, southeastern Brønnøya. The clasts have δ13C values of 4.18 ‰ and 5.90 ‰ (samples BD5 and BD6 in Fig. 12), clearly indicating a Hirnantian age for the conglomerate.

between the oolitic sandstone (Pilodden Member) and the brown based on lithology (Bockelie et al. 2017;units8and9representing siltstone, over which boundary there is a negative jump in δ13C the brown siltstone unit and the nodular limestone (Brønnøya values exceeding 3 ‰ (Fig. 15a), fairly similar to the correspond- Bed), respectively). The two lowermost carbon isotope samples ing transition at Konglungø. In May 2016 we returned to collect from this section (H12 and H13) were taken in the strata immedi- rock samples for δ13C analysis from a number of levels through- ately below the incised valley fill (Fig. 14b) and gave δ13Cvaluesof out the adjacent incised valley fill (Figs 14, 15b); starting just west 2.6 ‰, suggesting an early Hirnantian age for the youngest pre- of the type section and moving further west into the valley fill in served strata under the deepest part of the incised valley. Two order to cover the missing strata at the type section unconformity. samples (H1 and H2) from basal conglomerate clasts of oolitic These valley fill strata have been subdivided into nine subunits sandstone c. 5 m west of the diabase dike west of the type section

Fig. 11. Log profile and carbon isotope stratigraphy of the Pilbogen bay locality showing the rising limb of the HICE in the upper part of the Husbergøya Formation. The oolitic sandstone is overlain by shales of the Solvik Formation, and, although the latter formation outcrops in near proximity, the contact is not readily exposed at the locality. In overall terms the stratigraphy much resembles that at nearby Konglungø. Note, however, the different thicknesses of the Skaueren and Høyerholmen members as compared to their thicknesses at Konglungø, suggesting locally complex depositional patterns.

of HICE is preserved in the valley fill succession immediately overlying the conglomerate.

4.d. Stratigraphy and sedimentary facies at Vettre road-cut Vettre road-cut represents the top of an anticline, and the strata are near horizontal and well exposed on the east side of Road 165 (Fig. 17a–b; Bockelie et al. 2017, fig. 16). The lower portions of the section constitute clean oolite grainstone. The absence of quartz in this limestone stands in contrast to the oolitic sandstones or sandy oolites that are more common in the Hirnantian of the Oslo area; this is certainly due to the westerly position of Vettre, closer to the carbonate-dominated environments. The base of the oolite grainstone is not exposed, so its thickness at this locality is unknown. The oolite grainstone is conformably overlain by a bioclastic grainstone, which in turn is sharply overlain by a thin unit of hummocky cross-stratified, fine-grained sandstone interbedded with shale (Fig. 17a–b) indicating a deepening of the depositional environment. The upper part of the section constitutes brown, trough cross-laminated siltstone and sandstone and was interpreted as corresponding to the Brønnøya Bed on Hovedøya by Bockelie et al.(2017), i.e. as the later stages of the transgression that filled the incised valley there. The Solvik Formation is exposed on the opposite (west) side of the road, and field relationships suggest that it previously also followed on top of the brown, cross- bedded siltstone unit.

4.d.1. Carbon isotope stratigraphy at Vettre road-cut Twenty-seven carbon isotope samples from Vettre road-cut are presented here (Fig. 18a; Appendix 1). The oolite grainstone and most of the overlying bioclastic limestone have high δ13C values, mostly scattering between 5 and 5.8 ‰. A sudden decline by more than a 1 ‰ occurs in the upper c. 0.5 m of the bioclastic limestone. More samples are necessary to evaluate the possible significance of this negative jump. δ13C values continue to decrease in the over- Fig. 12. Log profile and carbon isotope stratigraphy at Store Ostsundet section, southeastern Brønnøya. Note high δ13C values in the oolitic sandstone clasts (samples lying calcareous sandstone and shale unit, and then return to val- BD5 and BD6). ues exceeding 5 ‰ again in the overlying Brønnøya Bed. A direct correlation with the Brønnøya Bed at Hovedøya, which has δ13C ‰ gave δ13Cvaluesof4.2‰ and 4.1 ‰.Twosamples(H14and values near 3 , is thus not confirmed by carbon isotope HOV24) from a similar conglomerate clast further west along geochemistry. the base of the incised valley gave δ13Cvaluesofc. 5.4 ‰ and 4.1 ‰, respectively. These four values are consistent with the 4.e. Stratigraphy and sedimentary facies at Olledalen oolitic sandstone of the Pilodden Member at the type section fur- skytebane ther east, at Konglungø, and with the peak values of the HICE in This is a c. 5 m thick and partly overgrown section measured at the the area. Hence, not only lithological similarity and stratigraph- Olledalen shooting range west of the village of Sem (Fig. 1). The ical relationships, but also geochemical evidence, proves the clasts strata are in near-vertical position and younger towards the north- to be reworked parts of the Pilodden Member, which is wide- west. The base of the section is at a weathered dolorite dike. Of par- spread in outcrops in the Oslo Fjord area. Above the conglomer- ticular stratigraphic importance in this section is the occurrence of ates, within the incised valley fill our carbon isotope data clearly the pentamerid brachiopod Holorhynchus giganteus. Based on δ13C show a falling trend from top values between 4.3 and 4.4 ‰ (sam- analysis and chitinozoan assemblages from other localities in the ples H19, H21, H22) in the lower portions to values between 1 and Asker district, it was concluded to be of latest Katian age by 2 ‰ in the brown siltstone (unit 8 of Bockelie et al. 2017). A sud- Brenchley et al. (1997). This brachiopod occurs a few decimetres den rise of values to c.3‰ occurs in the nodular limestone unit above the dike at Olledalen (Fig. 17c), implying a Katian age for the (Brønnøya Bed) that marks the top of the incised valley fill at the lower portions of the exposed section (see also Bockelie et al. 2017). pointwhenmosttopographywaslevelled.Abovetheincisedval- These parts constitute fine-grained clastic rocks with calcareous ley fill, in the type section, the values continue to decline to 1.2 ‰ concretions overlain by a thin limestone–marl alternation. This and 1.3 ‰ (samples HOV22–23 in Fig. 8). In summary, (1) the in turn is overlain by marly limestone and above this is a clean bio- erosion associated with the incised valley on Hovedøya never cut clastic limestone unit that is c. 2 m thick (Fig. 17d). The limestone into Katian strata, (2) oolitic conglomerate clasts at the bottom of ends the section upwards due to faulting and cut-out of strata, but the valley fill are lithologically identical to the Pilodden Member laterally it is clear that it is overlain by the Solvik Formation and also record Hirnantian δ13C peak values, providing evidence although the contact between the two units is not visible where for the timing of erosion, and (3) the major part of the falling limb our samples were taken.

Fig. 13. (Colour online) Photoplate showing stratigraphy and lithology of the overturned Ordovician–Silurian stratotype section at Hovedøya (eastern part). (a) The top of the Langøyene Formation at the classical and often-visited Ordovician–Silurian boundary stratotype locality (for additional information see Calner et al. 2013, stop 23). Here, grey oolitic sandstone is unconformably overlain by a brown siltstone unit and a nodular limestone unit (the ‘transitional beds’ in the early literature and now defined as the Kalvøya Member and Brønnøya Bed, respectively, by Bockelie et al. 2017). Above the nodular limestone unit is a sharp transition to the black shales of the Solvik Formation. The base of the nodular limestone unit was defined as the Ordovician–Silurian boundary by Brenchley & Newall (1975, p. 253; their top ‘Stage 5’). The boundary has later been shown to be higher up in the Solvik Formation (see text), but δ13C values of 1.17 ‰ (sample H23) and 1.26 ‰ (sample H22) in the thin limestone horizons of the basal Solvik Formation support a Silurian age also for this part (see also Fig. 15). (b) Top of the Langøyene Formation west of the section depicted in (a). Here the brown siltstone unit and the nodular limestone unit are developed in a very similar way to at the section described above. Note, however, how the light-coloured oolitic sandstone unit (Pilodden Member) is eroded away and only exists as conglomerate clasts several metres above the nodular limestone unit (Brønnøya Bed). Note distinct thickening of the the Kalvøya Member. The corresponding facies represents the peripheral wing of the incised valley further to the west.

4.e.1. Carbon isotope stratigraphy at Olledalen skytebane upper Ordovician section in Osmundsberget Quarry of the Thirteen carbon isotope samples were taken from the section at Siljan area (central Sweden), which falls in the interval just before Olledalen (Fig. 18b; Appendix 1). The level with H. giganteus is the start of the rising limb of the HICE (Ebbestad et al. 2015, fig. 9). bracketed by two δ13C samples (OSA1 and OSA2), both having val- Over the next 3 m, δ13C values increase steadily to a top value of 2.8 ues of 0.44 ‰, i.e. pre-Hirnantian values (Fig. 17c). This compares ‰ before the section ends at the fault plane. Thus the base of the well with the stratigraphic level of the Holorhynchus beds in the Hirnantian is very close to the last occurrence of H. giganteus and

Fig. 14. (Colour online) Photoplate showing stratigraphy and lithology of the overturned Ordovician–Silurian stratotype section at Hovedøya (western part). (a) A major erosional unconformity is prominent along the southwestern shore of the island. The incised valley fill was subdivided into nine subunits by Bockelie et al.(2017) and sampled for carbon isotope stratigraphy in the present study. (b) Position of isotope samples 12 and 13 just beneath the deepest cut of the incised valley. Relatively high δ13C values of these samples imply that the topmost beds underlying the incised valley fill, i.e. lower portions of the Langøyene Formation, are still lower Hirnantian. (c) Reworked conglomerate slabs of light- coloured oolitic sandstone of the Pilodden Member, the largest c. 1.5 m across, in the lower portions of the incised valley fill. (d) Reworked clasts of the Pilodden Member within the incised valley fill.

most of HICE is cut out at this locality, only the lower parts of the for early detailed studies of similar facies in the Oslo area; see also rising limb being preserved. The steady increase in δ13C values cor- Johnson & Baarli, 2018). The temporal series of primary sedimen- responds to a change in facies from argillaceous limestone to a tary structures within individual storm beds along with the fre- clean bioclastic limestone. quency and thickness of such beds have frequently been used as bathymetric indicators and for interpreting proximality trends in shelf successions (Brenchley et al. 1979; Dott & Bourgeois, 5. Sedimentary facies, environments and sea-level cycles 1982; Brenchley, 1989). In a facies model produced by Bockelie et al.(2017) briefly discussed and presented a sea-level Brenchley (1989), individual storm beds were subdivided into divi- curve for the studied interval. Their sea-level curve included three sions B, P, H, F, X and M. These divisions reflect the successive regressive cycles, of which the last lowstand had the most profound changes in current regime and influence from waves during the effect on the shallow marine environments. We herein add new build-up, peak and waning of an idealized storm event and the stratigraphic information, detail the sedimentology of the strata, associated rip current that transport sediment from the foreshore and expand on their interpretation to provide a highly resolved and shoreface areas into the lower shoreface, where the resulting Baltic perspective on the latest Ordovician sea-level oscillations. deposits rest largely protected from the post-storm, normal The major sedimentary lithofacies of the studied interval weather wave and current activity. The B (= basal scoured surface includes shale, limestone–marl alternations, shale – calcareous- with tool marks) and P (= massive, normally graded or planar siltstone alternations, massive siltstones, laminated or cross- laminated) divisions reflect the erosion and rapid deposition from laminated sandstones, as well as pure bioclastic and cross-bedded traction deposits, respectively, during the rip current. The H divi- limestones or mixtures of limestone and coarse sand. A majority of sion (= hummocky and swale zone) is the result of combined the siltstone and sandstone beds are typical storm deposits (tem- decelerated current and strong wave action. The waning storm pestites; see Brenchley et al. 1979 and Brenchley & Newall, 1980, and post-storm depositional process give rise to the F (= flat

Fig. 15. Log profile and carbon isotope stratigraphy of the Hovedøya sections. (a) The stratotype section on southern Hovedøya (loc. Hovedøya 1 of Bockelie et al. 2017). (b) Carbon isotope stratigraphy of the incised valley fill west of the stratotype section (loc. Hovedøya 2 of Bockelie et al. 2017). The sedimentology and palaeontology of units 1–9 are described in detail by Bockelie et al.(2017).

laminae, suspension material) and X (= cross-lamination) divi- thin and usually only weakly laminated and/or bioturbated. sions, whereas the M (= mudstone) division represents the finest They are few per given distance of stratigraphy, and lack any sub- grain sizes introduced by the current and/or the fair-weather back- stantial scouring, massive fabric and the hummocks and swales ground sedimentation between storm events. typical for the B-P-H divisions. We therefore interpret these as The overall lithofacies succession at Konglungø together with deposited by weak currents carrying mostly suspension material the above facies model for storm deposits allow for interpretation below the depth of wave influences during the late stages of storms, of depositional depth along an idealized shelf profile including the meaning at several tens of metres of depth (outer areas of the lower foreshore, shoreface, lower shoreface and offshore areas. Based on shoreface or even the offshore area, that is, near or below the storm our observations we identify four regressions of the sea-level, weather wave base, SWWB). Such relatively deep depositional herein named R-I to R-IV, including two superimposed higher- environment is further supported by the small lateral facies varia- order regressions (R-IIIA and R-IIIB; Fig. 19). tion of this unit and uniform distribution across the Oslo area. This siltstone tempestite facies grades conformably upwards into a nodular limestone–marl alternation indicating a transition to a 5.1. Regression 1 (R-I) shallower depositional environment, geographically closer to car- The siltstone tempestites of the lower Spannslokket Member (4dβ) bonate-producing environments. A similar facies transition, from of the Skogerholmen Formation are interbedded with shale, very siltstone tempestites interbedded with shale to a limestone–marl

Fig. 16. (Colour online) Profile from the southern shore of Hovedøya showing the geometry and sedimentary fill of a major incised valley, cutting into the lower portions of the Langøyene Formation. The fill represents the Kalvøya Member and is overlain by the more widespread Brønnøya Bed of the Solvik Formation. The valley fill represents an important sedimentary archive and time missing along the unconformity at the classical Ordovician–Silurian boundary stratotype section further to the east, where only the wing of the valley fill is preserved. Bockelie et al.(2017) subdivided the fill into nine units, of which only units 8 and 9 are represented at the classical type section. Sample points for carbon isotope stratigraphy in the present study are shown (H1–H26 þ HOV24; see also Fig. 15 and Appendix 1). Profile is modified from Bockelie et al.(2017).

alternation, in the overlying Husbergøya Formation, is associated shallowing is concurrent with a faunal change that similarly indi- with a change from deeper to shallower marine brachiopod faunas cates shallowing of the depositional environment (Brenchley & (Brenchley & Cocks, 1982). The above imply that limestone–marl Cocks, 1982). alternations developed landward of the facies belt with shale and The facies of this cycle starts with the basal shaly facies of the thin siltstone storm beds. The lack of wave-induced sedimentary lower Husbergøya Formation (Fig. 4c). It continues upward with a structures in the limestone–marl alternation suggests deposition successive increase in silt- or fine-grained sandstone storm-bed in the mid-shelf lower shoreface environment below the fair- frequency and thickness, before the facies transitions again into weather wave base (FWWB). Thus, a change from shale and a limestone–marl alternation in the higher parts of the formation. siltstone facies to limestone–marl alternations implies a small to The full transition is unfortunately obscured at Konglungø due moderate shallowing of the depositional environment. The oppo- to beach cover, but a nearby temporary excavation (along the site stratigraphy implies a similar deepening of the depositional strike) due to construction in 2016 suggests there are no major environment. With this rationale the transition from the deviations in the facies in this covered interval. The upper lime- Hovedøya (4dα) to lower Spannslokket (4dβ) members in the stone–marl alternation shows an upward increased frequency Skogerholmen Formation is interpreted as reflecting a relative and thickness of limestone beds, suggesting further shallowing. deepening of the depositional environment (from lower shoreface Establishment of a conspicuously shallow depositional environ- to offshore) and associated facies belt translocation. The 4dα–4dβ ment is supported by the simultaneous appearance of shelly fauna boundary is therefore marked as a flooding surface in Figure 19. and corals around −21 m in the section at Konglungø (Fig. 8). The upwards transition from the shale and siltstone facies of There is still no evidence, however, of sedimentary structures 4dβ to the limestone–marl alternation in 4dγ thus is interpreted denoting deposition above the FWWB. The significance in terms as a first, fairly moderate regressive cycle in the studied interval of relative sea-level of the overlying brown bioturbated sandstone (R-I in Fig. 19). unit (BBSU/D2 in Fig. 19) is not clear. This is an important marker bed in the area, but the lack of primary sedimentary structures, due to homogenization through bioturbation, and overall little facies 5.2. Regression 2 (R-II) variation, makes a depositional depth estimate difficult. Overall The facies shift from the nodular limestone–marl alternation in the it suggests lower depositional rates and a stable depositional envi- uppermost Skogerholmen Formation to shale in the Lower ronment, probably near the FWWB. A more precise estimation of Husbergøya Formation is especially pronounced, not only at depositional depth is not possible. The overlying sharp-based lime- Konglungø but throughout the area. At this level the exceptionally stone (unit D1) is a packstone to grainstone with abundant shelly rhythmic limestone–marl alternation of the Skogerholmen fauna of shallow-marine origin (Fig. 5) and signals continued shal- Formation is abruptly replaced by dark shales of the lower lowing to a depositional environment above the FWWB. Based on Husbergøya Formation, marking a clear transgressive pulse and the concurrent distinct shallowing and quickly rising δ13C values establishment of a deep depositional environment following R-I. through the upper Husbergøya Formation, along with the last Within the middle and upper Husbergøya Formation follows a appearance datum (LAD) of Tretaspis sp. and the first appearance clear progradational trend that represents R-II. This overall datum (FAD) of Hirnantia Fauna in the brown bioturbated

Fig. 17. (Colour online) Photoplate showing stratigraphy and lithology of Hirnantian strata at Vettre road-cut and at Olledalen skytebane, Asker district. (a) The section at Vettre with a transition from clean oolite and bioclastic grainstone to clastic deposition. The higher portion was interpreted as corresponding to the Brønnøya Bed by Bockelie et al. (2017), which is not supported by carbon isotope data of the present study. δ13C values above 5 ‰ rather suggest this is slightly older deposits of the Kalvøya Member, although it cannot be excluded that higher parts in the section corresponds to the Brønnøya Bed. (b) Detail showing Kalvøya Member (as interpreted herein) at Vettre, showing large-scale trough cross-bedding. (c) The lower portions of the section at Olledalen skytebane, showing the position of Holorhynchus giganteus (H) and the three isotope samples that support a Katian age for this brachiopod. (d) The higher portions of the section at Olledalen skytebane, showing the transition from argillaceous limestone to clean limestone (bioclastic grainstone). The steady rise of δ13C values through this interval from c. 0.5 ‰ to c. 2.7 ‰ supports an earliest Hirnantian age for the clean bioclastic limestone.

Fig. 18. Log profiles and carbon isotope stratigraphy of (a) Vettre road-cut and (b) Olledalen skytebane sections. The drop in δ13C values from above 5 ‰ to below 4 ‰ in the uppermost decimetres of the Pilodden Member at Vettre road-cut is intriguing but does not represent the falling limb of HICE since δ13C values rise above 5 ‰ again in the overlying unit. This latter unit is herein interpreted as part of the Kalvøya Member (incised valley fill), and its lower contact thus represents the sequence boundary. Diagenesis along this boundary may explain the lower δ13C values.

13 KONGLUNGØ SECTION Ccarb (‰) BATHYMETRY (without scaling) 0 1 23456‰ m OFF LS SF FS LAND

SOLVIK FORMATION A Clorinda association 0 FSST R-IV Pilodden Member B Brevilamnuellela Association L (oolitic limestone) TST A N FSST G B Ø Top Cycle 3 -5 R-III Y Høyerholmen C1 Dalmanella Association of Bockelie et al. E Member (2017), LOGC3 of N Ghienne et al. (2014) E Top Cycle 2 and Regression IV of of Bockelie et al. F Kiipli & Kiipli (2020) (2017), LOGC 3 LANGÅRA O C2 Cliftonia-Hindella Association of Ghienne et al. FORMATION -10 R (2014) and M A A Regression III T Dalmanella Association (Høyerholmen) and I Skaueren C3 II (Skaueren) of O Member Kiipli & Kiipli (2020) N -15 Hirnantia fauna HICE

LANGÅRA D1 Cliftonia-Hindella Association TST FORMATION FLOODING SURFACE BBSU D2 FAD Hirnantia Fauna HIRNANTIAN -20 LAD Tretaspis sp. FSST R-II

D3 Onniella Association Top Cycle 1 of Bockelie et al. -24 (2017) Covered interval, note scale change Regression I of -30 Kiipli & Kiipli (2020). Higher-order cycle of LOGC 3 HUSBERGØYA of Ghienne et al. 1 FORMATION D4 1 (2014)

-35 TRANSGRESSION

REGRESSION Tretaspis fauna

-40 E MFI TST FLOODING SURFACE TST - TRANSGRESSIVE SYSTEMS TRACT R-I -45 FSST - FALLING STAGE SKOGERHOLMEN SYSTEMS TRACT FORMATION F1 Spannslokket MFI - MAXIMUM FLOODING Member INTERVAL Higher-order (4dγ) SUBAERIAL -50 UNCONFORMITY cycle of LOGC 2 of Ghienne et al.

KATIAN (2014)

SKOGERHOLMEN FORMATION Spannslokket -55 F2 MFI Member (4dβ) TST FLOODING SURFACE Spinachitina cf. taugourdeaui F3 at approximately this level -60 in the Hovedøya section SKOGERHOLMEN ’lower HICE’? FORMATION Hovedøya Member Belonechitina gamachiana (4dα) at approximately this level in the Hovedøya section 2 -65 2

Fig. 19. Summary diagram of the Konglungø section showing formations, subunits A–F, main sedimentary facies, carbon isotope stratigraphy (including the development of HICE) and inferred relative sea-level change (regressions R-I through R-IV). The numbers ① and ② in the left margin denote the base of the Hirnantian Stage using carbon isotope stratigraphy and brachiopod data (① this study) or the appearance of the chitinozoan B. gamachiana in the succession (② Amberg et al. 2017), respectively. Correlation of the sea-level trend with sections in Morocco and Anticosti (Ghienne et al. 2014) and the East Baltic area (Kiipli & Kiipli, 2020) is tentative and hampered by uncertainties with the position of the lower Hirnantian Stage boundary, by the lack of graptolite biostratigraphy in the section, and by different scales of sea-level cycle hierarchy between the studies. Note that R-III is subdivided into two higher-order regressions

named R-IIIA and R-IIIB, with a minor intervening transgression. Here, at Konglungø, the falling limb of HICE is cut out at a major unconformity, the same unconformity that is associated with incised valley fills at Hovedøya and Brønnøya. Note interfingering of clastic vs calcareous facies in the Langøyene Formation, and thus also recurrent lithostratigraphy and brachiopod associations. BBSU = brown bioturbated sandstone unit (an important marker bed; see text). Sea-level curve: OFF = offshore, LS = lower shoreface, SF = shoreface, FS = foreshore.

sandstone unit (Brenchley & Cocks, 1982), we infer that the change difference in bedding, however, between the lower (Skaueren in bathymetry through this second regression (R-II in Fig. 19) Member) and upper (Høyerholmen Member) part of the reflects the first pronounced Hirnantian glacial and expansion Langøyene Formation. The lower member shows some interbed- of ice sheets across Gondwana. The maximum lowstand of this ding of shale or siltstone whereas the upper member shows only regression is inferred to be at the top of the Husbergøya amalgamated beds of sandstone. The preservation of fair-weather Formation, whereas the overlying limestone of the Langåra suspension material (background sedimentation; M division of Formtion (D1) marks a transgressive pulse. The latter unit also Brenchley, 1989) in the lower member suggests a depositional envi- has the highest δ13C values measured in this study (Fig. 9). ronment with high supply of sand but still below the FWWB; that The shallowing within R-II takes place over a limited thickness is, in the inner part of the lower shoreface. The lack of such shale of stratigraphy (˜20 m) and reflects a change from very deep off- interbeds in the upper member could theoretically be due to more shore deposition in the lowermost portions of the Husbergøya intense storms and thus erosion of the preceding fair-weather Formation to inshore deposition at the top. The top also marks muds, but fairly rapid progradation and further shallowing is a temporary halt in clastic deposition and a shift to clean carbonate the favoured interpretation here as this member is unconformably deposition (D1). Based on the overall strong facies asymmetry overlain by trough cross-bedded oolitic sandstones of the Pilodden within R-II along with the appearance of shelly fauna and corals, Member that was clearly deposited under the influence of strong we interpret it provisionally as a forced regression in which we currents above the FWWB. place the overlying sequence boundary at the top of the BBSU. We interpret the clastic facies succession of the Langøyene It should be noted that there is no firm sedimentary evidence Formation as one regressive cycle (R-III), but there are reasons for a forced regression, such as facies offset to Walters Law or a to discern two superimposed higher-order cycles (R-IIIA and R- submarine erosional surface such as a basal surface of forced regres- IIIB) formed in successively shallower water. The reason is twofold. sion or regressive surface of marine erosion (cf. Catuneanu, 2006). First, based on an erosional surface at the base of the Høyerholmen The base of the sandy BBSU potentially marks one of these surfa- Member at the type locality Skogerholmen, Bockelie et al.(2017) ces, but intense bioturbation and homogenization of the unit may interpreted a short-lived regressive event within the Langøyene have obliterated evidence for such a surface. Similarly, there is no Formation. Secondly, even if there is no evidence of a coeval ero- firm evidence of subaerial exposure in association with this boun- sional boundary at Konglungø, there is a break in clastic deposition dary, although the marked shift to clean carbonate deposition also at Konglungø in the form of a thin limestone wedge of the above suggests a major change in deposition across this surface. Langåra Formation separating the Skaueren and Høyerholmen members. This suggests a short-lived transgressive pulse when clastic deposition was briefly halted. The combined information 5.3. Regression 3 (R-III, including R-III and R-III ) A B leads us to include two higher-order sea-level cycles in the clastic Abovethelimestonewithshellyfauna and corals (D1), the depositio- facies of the Langøyene Formation (R-IIIA and R-IIIB), each one nal environment changes drastically throughout the Oslo–Asker dis- overlying a transgressive limestone wedge of the Langåra trict, and coarser clastic deposition prevails for the first and only time Formation (Fig. 19). in the Ordovician. The deposits form a major part of the rather heterogeneous Langøyene Formation and mirror the unusually shal- 5.4. The maximum lowstand of R-III and transgressive oolites low depositional environments that were established due to expand- ing ice sheets in Gondwana. Bockelie et al.(2017)suggestaminor The transition from the thick-bedded Høyerholmen sandstone to the transgression at the base of the Langøyene Formation based on the calcareous and oolitic facies of the Pilodden Member (Fig. 7a–b) is presence of a basal thin unit of micritic facies in the Skaueren particularly intriguing in terms of sea-level and can be interpreted Member at the type locality Skogerholmen. A minor transgression in two ways. This is either a submarine regressive surface of marine is needed to accommodate the thick sandstone units in the already erosion (RSME) or it is a subaerially exposed, erosional sequence shallow water reflected by the underlying limestone (D1). This was boundary (SB). The slight facies offset associated with the boundary inferably a short-lived event of minor scale (partial deglaciation) and the fairly continuous δ13Cvalues(offsetbyc. 1 ‰) across the and we include the underlying limestone (D1) in this transgression. boundary may indicate fairly continuous sedimentation and thus A more significant transgression would drown source areas, flatten the onset of a forced regression and a shift to carbonate-dominated the depositional equilibrium profile and turn any nearby deltas into deposition. There are, however, several aspects of the regional geology estuaries, locking up sediments at shorelines, and result in starved sed- and stratigraphy, as well as principles of sedimentation dynamics, that imentation throughout the Oslo area. The coarser clastic facies in the reduce the probability of such change. First, a lowered base level would Langøyene Formation is, on the contrary, only likely if relative increase gradients and produce even more sand to the depositional sea-level is further lowered, resulting in increased gradients and environment. It would not increase carbonate production in an prograding deltas, or if there is a stillstand of sea-level that permits already siliciclastic-dominated setting. Secondly, the highly calcareous progradation of coastal deltas into the basin. oolitic facies has a thin blanket-geometry in the central Oslo area. The tempestites in the Langøyene Formation differ substan- Such high-energy carbonate units typically develop during the trans- tially from those of the older units. Langøyene tempestites are gressive systems tract when siliciclastic source areas are drowned, dep- thick, coarsely sandy and show well-developed scouring, larger- ositional equilibrium profiles are of low-gradient type and low-relief scale low-angle cross-bedding and also slump structures (Fig. 6). shallow-water depositional systems are established. Third, oolites are The slump structures indicate that the gradient in the depositional much more likely to form during transgression when water-masses environment was fairly steep and the sands were likely redeposited transgress low-relief areas so that shallow-marine environments from delta front environments that supplied the sand from the east. expand, and water-masses are heated and release CO2,e.g.inthe In general, the individual sandstone beds are either massive or nor- aftermath of glacials. This principle is common to many low-latitude mally graded and show well-developed planar lamination and/or oolite deposits (Calner, 2005)aswellasforpre-Cambriancap hummocks and swales (i.e. the B-P-H zones; Fig. 6). There is a carbonates.

For the above reasons, the erosional boundary between the to collapse of the Gondwanan ice sheet. It should be noted that the Høyerholmen and Pilodden members is interpreted as a subaerial variation in brachiopod benthic assemblages in the basal Solvik unconformity marking the end of regression number 3 (R-III in Formation, in different localities across the Oslo–Asker district, Fig. 19) and principally a bypass surface and sequence boundary. shows that some of the underlying topography must still have Any lowstand deposits are not preserved in the outcrop area but existed as islands during the corresponding time (cf. Johnson & would be preserved further down-dip. As opposed to R-I and R- Baarli, 2018). II, which were not associated with subaerial exposure in the immediate study area, the lowstand of R-III exposed previous shelf deposits and, at least locally, removed shoreface deposits. 6. Discussion The calcareous oolitic facies of the Pilodden Member shows – fairly large-scale trough cross-bedding and represents a shallow- The Katian Hirnantian stratigraphy of the upper Oslo Fjord area water high-energy depositional environment, locally with dune has until now been based primarily on biostratigraphy. The carbon formation. It includes abundant reworked quartz (millet-seed isotope stratigraphy presented here and the identification of the quartz), well-abraded fossil fragments and corals as well as clasts HICE therefore further resolves previous stratigraphic data and of the underlying sandstones. The millet-seed sand is interpreted better relates the various stratigraphic units, the shifts in sedimen- as being the result of reworking of nearby terranes during a more tation patterns, and thus relative sea-level, to the uppermost Katian pronounced late Hirnantian lowstand. With the above reasoning and Hirnantian stages. This potentially enables correlation with we infer that the Pilodden Member was deposited during a rapid several tens of Hirnantian localities globally. It is beyond the scope transgression (deglaciation) in the later part of the Hirnantian. A of this paper, though, to attempt such correlations here. transgressive origin for this unit is also supported by observations from the island Høyerholmen (K Tonstad, unpub PhD thesis, 6.1. Summary of δ13C stratigraphy and the base of the Univ. Bergen, 1983, p. 206). Hirnantian Stage There is no proper definition of the beginning and end of the 5.5. Regression 4 (R-IV) HICE. A slow rise in δ13C values from the baseline is evident within The regional erosional surface marking the upper boundary of the the M. extraordinarius Zone, e.g. at the Wangjiawan North Section Pilodden Member is the most conspicuous unconformity in the in Hubei province, China (Chen et al. 2006). The most rapid rate of studied sections. It is marked by an offset in the δ13C values exceed- increase in δ13C, however, appears to coincide with the base of the ing 2 ‰ at Konglungø (Fig. 9) and 3 ‰ at Hovedøya (Fig. 15). upper Hirnantian M. persculptus Zone (Bergström et al. 2013; Based on δ13C data and lithological similarity, it is clear that the Ghienne et al. 2014; Kröger et al. 2015; Mauviel & Desrochers major conglomerate clasts in the lower parts of the incised valleys 2016; Wang et al. 2019). This temporal pattern of change may help at both Hovedøya and Brønnøya (Bockelie et al. 2017) derive from to indicate preliminary positions for the base of the M. extraordi- the Pilodden Member. Hence, the sandy oolitic facies of this narius and M. persculptus zones, respectively. member must have been buried and lithified before a fourth regres- Among the studied sections, the Konglungø locality represents sion exposed it and it was subject to subaerial weathering and ero- the most complete record of the HICE (Figs 8–9, 19). Here, the sion. We herein refer to this shallowing as regression number 4 (R- onset and rising limb of HICE is associated with the upper IV in Fig. 19) and it cut deeper than any of the preceding regres- Husbergøya Formation and with the last and first appearances sions, locally to the depth of the Skogerholmen Formation of Tretaspis and Hirnantia Fauna, respectively. Below this interval, (Bockelie et al. 2017). Quartz sand, including millet-seed quartz, pre-excursion δ13C baseline values are between 0 and 1 ‰, corre- that could not be further weathered or dissolved during this low- sponding to the Spannslokket Member of the Skogerholmen stand, locally forms a transgressive lag concentration deposit on Formation. Slightly higher values just below this interval, within top of the Pilodden Member, e.g. the dark grey sandstone at the Hovedøya Member of the same formation, may represent Konglungø which then effectively would belong to the Kalvøya the falling limb of the ‘lower HICE’, a minor excursion that clearly Member (Fig. 9). predates the major Hirnantian δ13C anomaly, and which has been Carbonate rocks lithify comparably early due to the carbonate documented from Anticosti (Mauviel & Desrochers, 2016) and saturation state of sea-water, and may in warm and dry climates possibly also from Mirny Creek (Kaljo et al. 2012). In Anticosti this lithify within a few thousand years (e.g. the Holocene oolites of ‘early HICE’ is associated with the chitinozoan Belonechitina the Bahamas). In order to lithify, the Pilodden Member must have gamachiana. Hence, the recent findings of B. gamachiana also been overlain by at least some succession of strata that is no longer in the Skogerholmen Formation at Hovedøya (Amberg et al. preserved in the Oslo–Asker district. R-IV removed these deposits 2017) appear to support the presence of the ‘lower HICE’ in and led to the development of pronounced topography and several Baltica as well, although additional sampling is needed to fully sup- confined, incised valleys in the area (Bockelie et al. 2017). The local port this. The findings of B. gamachiana are also intriguing since fills of these valleys were collectively grouped as the Kalvøya the eponymous biozone has been argued to be fully within the Member. Our δ13C data show that such valleys in Hovedøya Hirnantian in Anticosti (Delabroye & Vecoli, 2010; Amberg and Brønnøya filled during transgression already in the et al. 2017). As there is no proper solution to this stratigraphic Hirnantian (Figs 12, 15) and that also the overlying Brønnøya problem at the moment, we will rely on our brachiopod data Bed most likely was deposited in the Hirnantian as well. The and the onset of the HICE to provisionally set the Katian– Brønnøya Bed has a wider distribution in the Oslo–Asker district Hirnantian stage boundary at c. 15 m below the top of the than the immediately underlying and more scattered incised valley Husbergøya Formation (sample KON47; Fig. 8). In our compila- fills (Kalvøya Member) and thus marks the time when much of the tion of data and interpretations from Konglungø (Fig. 19) we have underlying topography was submerged. The overlying black shales indicated a second possible position for the base of the Hirnantian of the Solvik Formation mark the continued quick deepening due Stage based on alignment with Anticosti as guidance.

The stratigraphic interval containing the rising limb of the post-glacial transgression takes place in the latest Hirnantian. Even HICE is partly covered by modern beach deposits, hampering if overall similarities are striking, a detailed correlation of the sea- study of the entire morphology of the rise. Parts of the covered level cycles between Baltoscandia and Morocco is difficult and at interval are exposed, however, at the nearby Pilbogen Bay locality the current time hampered by lack of precise biostratigraphic con- at Brønnøya. Based on the combined set of observations from these trol; in neither area is it possible to exactly pinpoint the Katian– two localities, the rising limb of the HICE shows an increase from Hirnantian stage boundary or the M. extraordinarius/M. persculp- δ13C values near the baseline in the middle part of the Husbergøya tus boundary. Also, as discussed above, the range of the Hirnantian Formation to near 6 ‰ in the first wedge of the Langåra Formation Stage depends on whether the chitinozoan B. gamachiana is effec- (unit D1 in Fig. 9) some 18 m higher up. The rise occurs in beds tively within the stage or not. With our preferred definition of the assignable to the Onniella Association and continues into beds base of the Hirnantian in the Oslo–Asker district (No. 1 in Fig. 19) with typical Hirnantia Fauna (Fig. 19). This suggests that the rising our cycles R-I through R-IV inferably represent higher-order limb of the HICE is most likely confined to the middle–upper cycles superimposed on Late Ordovician Glacial Cycles 2 and 3 Hirnantian Metabolograptus persculptus Graptolite Biozone and, (LOGC 2-3) of Ghienne et al. (2014; Fig. 19). R-I and R-II are ten- as a result, that the M. extraordinarius Graptolite Biozone is very tatively correlated as parts of LOGC 2 and 3, respectively, which thin or missing in the studied sections. The preserved plateau of the includes uppermost Katian strata and most of the lower and HICE, which falls fully within the overlying Langøyene Formation, middle part of the Hirnantian. Both regressions R-III and R-IV thus appears to be entirely within the uppermost Hirnantian. This take place entirely within the high-δ13C-value ‘plateau’ of HICE, is further supported by the falling limb of the isotope excursion clearly after the end of the rising limb, but before the falling limb being located above the occurrence of the Brevilamnulella of the excursion (which is documented in transgressive deposits Association. It is also in alignment with data from the Boda within the incised valley formed during the maximum lowstand Limestone in central Sweden that show B. kjerulfi to occur in beds of R-IV). Both regressions therefore overlap with the stratigraphic assignable to the uppermost M. persculptus Zone (Rasmussen et al. range of LOGC3 of Ghienne et al. (2014). 2010; Kröger et al. 2015). This part of the curve is characterized by slightly lower δ13C values, and especially by a less stable signal with values varying by 3–5 ‰ in the calcareous sandstone facies. δ13Cis 7. Conclusions increasing again with less varying values in the more calcareous A dataset of 208 whole-rock carbon isotope samples from six sec- and oolitic Pilodden Member in the uppermost part of the tions in the Oslo–Asker district forms the basis for the first com- Langøyene Formation (where the Brevilamnulella Association prehensive documentation of the HICE in Norway. We relate this occurs). The slight and consistent shift to lower δ13C values, and dataset in detail to sedimentary facies and depositional depth and the less stable signal, is thus likely due to the dominantly siliciclas- present a sea-level history for the time interval. This reveals a long- tic facies and rhythmic sedimentation style of the Skaueren and term regression superimposed by four shorter-lived (fourth-order) Høyerholmen members of the Langøyene Formation. An almost regressive–transgressive cycles with a stepwise increase in erosion identical pattern is seen in the profound late Silurian Lau Event; of the shelf environment. A tentative correlation of these cycles in the associated δ13C excursion on Gotland, Sweden, a set-back with latest Katian–Hirnantian sea-level cycles documented from in the plateau values is associated with the Burgsvik Sandstone the East Baltic area (Kiipli & Kippli 2020) and Morocco and (Younes et al. 2017). The falling limb of the HICE is largely cut Anticosti (Ghienne et al. 2014) is proposed, but we note uncertain- out at Konglungø. This is related to the major unconformity at ties primarily due to limitations in biostratigraphy. The main con- the top of the Langøyene Formation. In this study we were able clusions of the study can be summarized as follows: to sample and document a major part of the falling limb in the incised valley fill associated with this unconformity at Hovedøya. 1) The HICE is identified at Hovedøya, Brønnøya, Konglungø, Vettre and Olledalen in the Oslo–Asker district. The HICE is best preserved at Konglungø where the onset of the rising limb 6.2. Provisional correlation of sea-level cycles starts c. 15 m below the top of the Husbergøya Formation and δ13 þ ‰ The studied interval is characterized by a series of four regressions where the anomaly reaches Ccarb peak values between 5 (R-I to R-IV) with progressively increased erosion of previous shelf and þ6 ‰ in the Langåra and Langøyene formations. The fall- deposits. A similar general pattern was presented by Kiipli & Kiipli ing limb is missing due to erosion at Konglungø but partly pre- (2020) in their review of Hirnantian sea-level changes in served in incised valley fills (Kalvøya Member) at Hovedøya, Baltoscandia. Based on sections in the East Baltic area, they pro- Brønnøya and Vettre. posed four major transgressive–regressive cycles through this 2) The Katian–Hirnantian boundary as expressed by the onset of interval. A tentative correlation between the Oslo–Asker district HICE and the first appearance of Hirnantia brachiopod fauna and the East Baltic area is included in Figure 19. If we adopt the is herein tentatively placed c. 15 m below the top of the recently suggested duration of the Hirnantian Stage as only 0.47 Husbergøya Formation at Konglungø. This decision is chal- ± 0.34 Ma (Wanhe section; Ling et al. 2019), our sea-level cycles lenged by the recent and local finds of the chitinozoan R-I through R-IV appear to be in the same range as Quaternary Belonechitina gamachiana (Amberg et al. 2017). If this taxon glacial–interglacial cycles (fourth order of c. 100 kyr) whereas R- is used as the basis for an intra-Hirnantian biozone, as in some – IIIA and R-IIIB would be superimposed higher-frequency cycles North American sections, the Katian Hirnantian boundary (fifth order). would need to be shifted significantly lower in the succession, In general terms, a similar development, also with progressively to within the Skogerholmen Formation; the preserved increased amplitude of sea-level falls, and a maximum glacio- Hirnantian succession is then more than 60 m thick, and the eustatic regression in the late or latest Hirnantian, is documented base of the Hirnantian situated more than 40 m below the from Morocco, representing a high-latitude Gondwana setting FAD of Hirnantia Fauna. It would also suggest that the main (Ghienne et al. 2014; Colmenar et al. 2018). As in Baltica, the main part of the Skogerholmen and Husbergøya formations belongs

in the M. extraordinarius graptolite Zone, since high δ13C val- Ainsaar L, Truumees J and Meidla T (2015) The position of the Ordovician– ues of the HICE globally appear to be restricted to the M. per- Silurian Boundary in Estonia tested by high-resolution δ13C chemostrati- sculptus Zone. graphic xorrelation. In Chemostratigraphy – Concepts, Techniques, and 3) Based on facies analysis and the presence of erosional uncon- Applications (ed M. Ramkumar), pp. 395–412. Amsterdam: Elsevier Science. formities, four sea-level falls are recorded, successively with Aldridge R, Jeppsson L and Dorning K (1993) Early Silurian oceanic episodes – increased erosion: A first minor regression (R-I) is associated and events. Journal of the Geological Society 150, 501 13. Amberg C, Vandenbroucke T, Nielsen A, Munnecke A and McLaughlin P with the upper Skogerholmen Formation. A second regression (2017) Chitinozoan biostratigraphy and carbon isotope stratigraphy from (R-II) is associated with the upper portions of the Husbergøya the Upper Ordovician Skogerholmen Formation in the Oslo Region: a Formation and corresponds to the rising limb of the HICE. R-I new perspective for the Hirnantian lower boundary in Baltica. Review of and R-II are tentatively correlated as overlapping with LOGC 2 Palaeobotany and Palynology 246, 109–19. and 3, respectively, in Gondwana and Anticosti sensu Ghienne Baarli BG (2014) The early Rhuddanian survival interval in the Lower Silurian et al. (2014), whereas R-II inferably correlates with Regression I of the Oslo Region: a third pulse of the end-Ordovician extinction. in the East Baltic area sensu Kiipli & Kiipli (2020). A third Palaeogeography, Palaeoclimatology, Palaeoecology 395,29–41. regression (R-III) is manifested by prograding sandstone units Bauert H, Ainsaar L, Põldsaar K and Sepp S (2014) δ13C chemostratigraphy of within the Langøyene Formation and eroded at least the fore- the Middle and Upper Ordovician succession in the Tartu-453 drillcore, shore area. Two superimposed fifth-order sea-level cycles are southern Estonia, and the significance of the HICE. Estonian Journal of – identified within R-III (R-III and R-III ) and correlated with Earth Sciences 63, 195 200. A B Bergström SM, Eriksson ME, Young SA, Ahlberg P and Schmitz B Regressions II and III in the East Baltic area, respectively. A (2013) Hirnantian (latest Ordovician) δ13C chemostratigraphy in southern fourth regression (R-IV) is evident from major incision through Sweden and globally: a refined integration with the graptolite and conodont the upper parts of the Langøyene Formation, resulting in chan- zone successions. GFF 136, 355–86. nels and valleys both in Hovedøya and Brønnøya. R-III and R- Bergström SM, Lehnert O, Calner M and Joachimski MM (2012) A new upper IV are tentatively correlated as overlapping with LOGC3 in Middle Ordovician–Lower Silurian drillcore standard succession from Gondwana and Anticosti sensu Ghienne et al. (2014). The Borenshult in Östergötland, southern Sweden: 2. Significance of δ13C chemo- transgressive strata that fill the incised valley formed during stratigraphy. GFF 134,39–63. R-IV record the falling limb of the HICE, dating the last major Bergström SM, Saltzman MR and Schmitz B (2006) First record of the regression and lowstand to the latest part of the international Hirnantian (Upper Ordovician) δ13C excursion in the North American Hi1 stage slice. The final post-glacial transgression thus occurs Midcontinent and its regional implications. Geological Magazine 143, – in the latest Hirnantian. 657 78. Bergström SM, Schmitz B, Young SA and Bruton DL (2010) The δ13C chemostratigraphy of the Upper Ordovician Mjøsa Formation at Furuberget near Acknowledgements. Johan Fredrik Bockelie passed away during the course of Hamar, southeastern Norway: Baltic, Trans-Atlantic and Chinese relations. this work. M.C. and H.C. had the opportunity to work closely with Fredrik in the Norwegian Journal of Geology 90,65–78. Oslo–Asker district in 2015 and 2016 and to learn from his lifelong experience Bergström SM, Xu C, Gutiérrez-Marco JC and Dronov A (2009) The new and astonishing knowledge of the geology of this area. We are grateful for his chronostratigraphic classification of the Ordovician System and its relations warm hospitality, and this paper is dedicated to his memory. We acknowledge to major regional series and stages and to the d13C chemostratigraphy. also Angie Bockelie for her hospitality and company during our field visits. We Lethaia 42,97–107. are greatly indebted to Knut Mansika and family for sharing their private garden Bockelie JF (1978) The Oslo Region during the Early Palaeozoic. In Tectonics at Konglungø on several occasions during our field works. Fylkesmannen in Viken county, Oslo, gave permission to study protected areas of Konglungø. and Geophysics of Continental Rifts (eds IB Ramberg and ER Neumann), pp. – Gudveig Baarli and Markes E. Johnson are acknowledged for providing the orig- 195 202. Dordrecht: D. Reidel. – inals of Figures 1 and 16 and for fruitful discussions on the stratigraphy of the Bockelie JF (1982) The Ordovician of Oslo Asker. In Field Excursion Guide. IV Oslo–Asker district. Enli and Tarmo Kiipli provided information for our ten- International Symposium on the Ordovician System (eds DL Bruton and SH – tative correlation of relative sea-level changes. This paper is to a great extent the Williams), pp. 106 21. Paleontological Contributions from the University of product of a sabbatical stay at the Department of Geology, Lisbon University, in Oslo 279. the autumn of 2018 and M.C. is grateful to Conceição Freitas for the excellent Bockelie JF (2013) Stop 23. Beach sections on Hovedøya (island in the Oslo working possibilities and office provided during this stay. We acknowledge the Fjord). In The Lower Palaeozoic of Southern Sweden and the Oslo Region, insightful and useful reviews by Gudveig Baarli, Jean-François Ghienne, Leho Norway. Field Guide for the 3rd Annual Meeting of the IGCP Project 591 – Ainsaar and Jan-Ove Ebbestad. C.M.Ø.R. acknowledges support from (eds M Calner, P Ahlberg, O Lehnert and M Erlström), pp. 82 5. Sveriges Geocenter Denmark (projects 2015-5 and 3-2017). 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Appendix 1. δ13C data from the six sections studied in the Oslo–Asker district of southern Norway

Konglungø Measured from reference level between the oolitic sandstone (Pilodden Member) and the overlying ‘millet-seed’ dark grey sandstone (Figs 8–9). Note doubled analysis of a few samples (labelled II). Sample Metres δ13C(‰) δ18O(‰) KON1 −3.22 m 4.23 −18.33 KON2 −2.95 m 5.04 −16.87 KON3 −1.86 m 5.07 −16.61 KON4 −1.15 m 5.09 −17.97 KON5 −0.83 m 5.15 −17.13 KON6 −0.13 m 4.62 −17.52 KON7 þ0.25 m 1.66 −18.84 KON8 þ0.22 m 1.86 −18.93 KON9 þ0.63 m 0.96 −17.76 KON10 þ1.1 m 0.83 −17.46 KON11 þ1.3 m 1.74 −16.35 KON12 þ0.2 m 2.58 −18.12 KON13 þ0.1 m 2.28 −16.85 KON14 −2.64 m 4.95 −16.17 KON15 −2.15 m 5.08 −16.12 KON16 −1.6 m 5.47 −16.37 KON17 −0.5 m 5.04 −16.83 KON18 −3.7 m 3.84 −18.41 KON19 −4.2 m 4.04 −18.32 KON20 −4.95 m 4.34 −18.44 KON21 −5.5 m 4.08 −16.93 KON22 −6.2 m 3.62 −19.01 KON23 −7.25 m 3.01 −16.80 KON23 (II) −7.25 m 2.98 −16.82 KON24 −8.25 m 4.65 −17.46 KON25 −8.75 m 3.89 −17.06 KON26 −9.85 m 5.25 −16.47 KON27 −11.1 m 4.19 −16.84 KON28 −12.1 m 5.30 −16.74 KON29 −13.1 m 4.73 −16.19 KON30 −13.8 m 4.98 −16.88 KON31 −14.8 m 4.52 −17.19 KON32 −15.6 m 5.17 −16.17 KON33 −16.4 m 3.81 −15.51 KON34 −16.9 m 5.68 −15.35 KON35 −17.1 m 6.01 −16.58 KON36 −17.6 m 5.88 −17.26 KON37 −18.0 m 5.54 −18.02 KON38 −18.4 m 5.69 −17.73 KON39 −18.7 m 4.30 −12.03 KON40 −19.35 m 4.26 −13.40 KON40 (II) −19.35 m 4.23 −13.46 (Continued)

Appendix 1. (Continued )

Konglungø Measured from reference level between the oolitic sandstone (Pilodden Member) and the overlying ‘millet-seed’ dark grey sandstone (Figs 8–9). Note doubled analysis of a few samples (labelled II). Sample Metres δ13C(‰) δ18O(‰) KON41 −20.65 m 4.67 −16.52 KON42 −21.95 m 3.60 −17.62 KON43 −23.25 m 2.49 −17.86 KON44 −30.65 m 0.88 −16.90 KON45 −31.65 m 0.49 −17.46 KON46 −32.8 m 0.70 −17.47 KON47 −33.85 m 0.25 −17.24 KON48 −35.25 m −0.19 −16.88 KON49 −42.75 m 1.09 −17.49 KON50 −43.20 m 1.10 −17.91 KON51 −43.85 m 1.13 −18.02 KON52 −44.55 m 1.01 −17.83 KON53 −22.07 m 3.48 −18.44 KON54 −22.22 m 3.63 −17.20 KON55 −22.42 m 3.31 −16.79 KON56 −22.62 m 2.68 −15.80 KON57 −22.82 m 3.03 −17.11 KON58 −22.97 m 2.89 −16.09 KON59 −23.07 m 2.87 −16.90 KON60 −23.27 m 2.61 −18.01 KON61 −21.60 m 3.91 −17.19 KON62 −21.12 m 4.36 −16.93 KON63 −20.7 m 4.63 −16.81 KON64 −20.37 m 4.76 −16.79 KON65 −19.37 m 4.87 −14.94 KON66 −18.67 m 5.15 −16.24 KON67 −18.47 m 5.44 −18.00 KON68 −18.27 m 5.73 −17.93 KON69 −18.03 m 5.60 −18.46 KON70 −17.83 m 5.77 −18.89 KON71 −16.83 m 5.46 −15.24 KON72 −57.35 m 1.12 −17.04 KON73 −57.75 m 1.40 −17.67 KON74 −58.9 m 1.55 −17.46 KON75 −59.6 m 1.31 −16.64 KON76 −61.2 m 1.39 −17.97 KON77 −55.65 m 0.59 −16.70 KON78 −55.3 m 0.68 −16.82 KON79 −54.65 m 0.46 −16.11 KON80 −52.9 m 0.29 −16.52 KON81 −52.1 m 0.22 −16.99 KON82 −51.15 m 0.14 −16.34 (Continued)

Appendix 1. (Continued )

Konglungø Measured from reference level between the oolitic sandstone (Pilodden Member) and the overlying ‘millet-seed’ dark grey sandstone (Figs 8–9). Note doubled analysis of a few samples (labelled II). Sample Metres δ13C(‰) δ18O(‰) KON83 −50.2 m 0.11 −16.25 KON84 −48.8 m 0.51 −17.65 KON85 −47.3 m 0.90 −17.23 KON86 −45.8 m 1.06 −17.56 Brønnøya, Pilbogen bay Measured from reference level at the top of the oolite at the bridge in the northern end of the section. Sample Metres δ13C(‰) δ18O(‰) BR-1 −0.55 m 5.18 −17.79 BR-2 −0.99 m 4.53 −16.59 BR-3 −1.62 m 5.19 −16.39 BR-4 −2.42 m 5.31 −16.79 BR-5 −2.72 m 5.23 −16.35 BR-6 −3.02 m 5.42 −16.11 BR-7 −3.47 m 5.44 −16.75 BR-8 −3.77 m 5.33 −16.40 BR-9 −3.87 m 4.43 −17.67 BR-10 −4.03 m 5.06 −16.65 BR-11 −4.37 m 5.27 −17.73 BR-12 −5.07 m 2.72 −17.16 BR-13 −5.97 m 4.69 −16.23 BR-14 −6.67 m 4.10 −17.88 BR-15 −14.5 m 4.79 −19.15 BR-16 −14.8 m 5.50 −18.74 BR-17 −18.37 m 4.89 −18.43 BR-18 −18.6 m 4.83 −18.63 BR-19 −20.1 m 4.02 −18.39 BR-20 −21.65 m 3.44 −18.30 BR-21 −22.80 m 2.55 −18.05 BR-22 −23.80 m 2.21 −18.02 BR-23 −24.42 m 1.67 −18.83 BR-24 −25.17 m 1.54 −18.67 BR-25 −25.84 m 1.83 −18.96 Brønnøya, Store Ostsundet Measured from reference level at the base of the limestone–marl alternation just north of the concrete bridge that lies 50 m southeast of the intersection of Vierndroget/Viernveien. Sample Metres δ13C(‰) δ18O(‰) BD1 −1.26 m 3.01 −19.89 BD2 −2.29 m 3.82 −20.52 BD3 −4.01 m 4.18 −20.56 BD4 −4.9 m 4.23 −19.48 BD5 −7.5 m 5.90 −20.07 BD6 −8.0 m 4.18 −20.55 (Continued)

Appendix 1. (Continued )

Brønnøya, Store Ostsundet Measured from reference level at the base of the limestone–marl alternation just north of the concrete bridge that lies 50 m southeast of the intersection of Vierndroget/Viernveien. Sample Metres δ13C(‰) δ18O(‰) BD7 þ0.42 m 3.27 −21.34 BD8 þ0.7 m 3.68 −21.36 Hovedøya, classical type section Measured from reference level at the top of the thick-bedded oolitic sandstone (Pilodden Member). Note doubled analysis of a few samples (labelled II). Sample Metres δ13C(‰) δ18O(‰) HOV1 þ1.17 m 2.38 −17.28 HOV2 þ1.09 m 1.56 −17.04 HOV3 þ1.02 m 2.73 −16.97 HOV4 þ0.97 m 2.93 −16.98 HOV5 þ0.94 m 2.95 −17.01 HOV6 þ0.87 m 2.95 −16.87 HOV7 þ0.83 m 2.58 −16.92 HOV8 þ0.79 m 2.35 −16.96 HOV9 −0,02 m 5.27 −13.57 HOV10 −0.26 m 4.63 −16.71 HOV11 −0.71 m 5.00 −16.73 HOV12 −1.69 m 4.61 −16.71 HOV13 −1.0 m 4.17 −16.13 HOV14 −1.5 m 4.53 −15.93 HOV15 −3.0 m 4.65 −16.18 HOV16 −2.13 m 4.49 −16.43 HOV17 þ0.14 m 1.93 −12.76 HOV17 (II) þ0.14 m 1.89 −12.70 HOV18 þ0.2 m 1.75 −11.75 HOV18 (II) þ0.2 m 1.78 −11.66 HOV19 þ0.41 m 1.66 −10.85 HOV20 þ0.64 m 1.54 −12.66 HOV21 þ0.04 m 1.90 −13.27 HOV21 (II) þ0.04 m 1.93 −13.25 HOV22 þ1.84 m 1.26 −16.57 HOV23 þ2.2 m 1.17 −16.85 HOV24 co.clast 4.11 −13.13 Hovedøya, incised valley Samples marked with * were measured from the basal unconformity just west of the big conglomerate block (>1 m) near the deepest part of the valley fill. The others were measured from various unit boundaries and along the width of the basin fill (see Fig. 16), and later recalculated to metres below the base of unit 8. This gave rise to uncertainties for the precise stratigraphic level of five samples, marked grey in Fig. 15. Sample Metres δ13C(‰) δ18O(‰) H1 co.clast 4.16 −14.19 H2 co.clast 4.07 −14.6 H3 unit 5 2.84 −14.41 H4 unit 5 2.58 −14.23 H5 −4.75 m 3.66 −15.19 H6 −4.1 m 2.87 −15.17 (Continued)

Appendix 1. (Continued )

Hovedøya, incised valley Samples marked with * were measured from the basal unconformity just west of the big conglomerate block (>1 m) near the deepest part of the valley fill. The others were measured from various unit boundaries and along the width of the basin fill (see Fig. 16), and later recalculated to metres below the base of unit 8. This gave rise to uncertainties for the precise stratigraphic level of five samples, marked grey in Fig. 15. Sample Metres δ13C(‰) δ18O(‰) H7 c. −3.8 m 2.17 −14.87 H8 c. −3.8 m 1.64 −15.46 H9 −3.45 m 2.2 −15.93 H10 −1.0 m 1.11 −15.7 H11 −0.1 m 1.73 −15.82 H12* −0.3 m 2.58 −12.05 H13* −0.01 m 2.63 −11.57 H14 co.clast 5.43 −12.45 H15 −10.9 m 3.41 −12.67 H16 −10.6 m 3.53 −9.84 H17 −10.1 m 2.22 −12.93 H18 −9.6 m 3.42 −11.62 H19 −9.4 m 4.4 −13.01 H20 −8.95 m 3.2 −13.36 H21 −8.05 m 4.36 −13.03 H22 −6.85 m 4.32 −12.53 H23 −6.15 m 3.43 −13.31 H24 −6.05 m 3.35 −12.08 H25 −5.5 m 3.06 −11.74 H26 −4.6 m 2.02 −12.53 Vettre road-cut Measured from reference level at the top of the clean limestone (clean oolite in lower parts) in the southern portion of the outcrop. Sample Metres δ13C(‰) δ18O(‰) VE1 −0.03 m 4.45 −12.06 VE2 −0.1 m 3.65 −10.71 VE3 −0.7 m 5.78 −10.50 VE4 −0.9 m 4.83 −11.59 VE5 −1.0 m 5.25 −11.46 VE6 −1.1 m 5.21 −11.95 VE7 −1.2 m 5.44 −11.31 VE8 −1.3 m 4.99 −11.45 VE9 −1.4 m 5.41 −11.54 VE10 −1.5 m 5.36 −11.26 VE11 −1.6 m 5.67 −11.23 VE12 −1.7 m 5.63 −11.12 VE13 −1.8 m 4.31 −11.11 VE14 −1.9 m 5.48 −12.00 VE15 −2.0 m 5.55 −11.29 VE16 −2.1 m 5.50 −12.21 VE17 −2.2 m 5.75 −11.31 VE18 −2.3 m 4.61 −12.79 (Continued)

Appendix 1. (Continued )

Vettre road-cut Measured from reference level at the top of the clean limestone (clean oolite in lower parts) in the southern portion of the outcrop. Sample Metres δ13C(‰) δ18O(‰) VE19 −2.4 m 5.80 −11.53 VE20 −2.5 m 5.11 −12.65 VE21 −0.5 m 4.94 −11.86 VE23 þ0.07 m 3.60 −10.78 VE24 þ0.18 m 2.31 −11.59 VE27 þ0.4 m 4.13 −11.88 VE28 þ1.0 m 5.28 −12.46 VE29 þ1.18 m 5.13 −12.19 VE30 þ1.68 m 3.33 −9.7 Olledalen skytebane Measured from the dolerite dyke. Sample Metres δ13C(‰) δ18O(‰) OSA 1 þ0.2 m 0.44 −12.00 OSA 2 þ0.24 m 0.44 −11.41 OSA 3 þ0.5 m 0.87 −13.04 OSA 4 þ0.84 m 0.77 −14.60 OSA 6 þ1.8 m 0.26 −12.90 OSA 7 þ2.49 m 0.54 −14.38 OSA 8 þ2.78 m 1.30 −12.15 OSA 9 þ3.08 m 1.62 −12.97 OSA 10 þ3.52 m 1.96 −11.48 OSA 11 þ3.66 m 2.35 −13.25 OSA 12 þ4.1 m 2.76 −13.86 OSA 13 þ4.8 m 2.62 −11.68